What should you do?
Call a judge.
In this instance, as an observant player, you should notice that he has mistapped his mana to play the Henchfiend. This can often be an innocent mistake, but in this case there are two things that mean it is unlikely. He has only one Mountain and did not tap it to play the haste creature, whilst on the board are two creatures that both need a single black, not red, mana to activate their ability. Secondly, if you block two of his men and let one through, you fall to 3 life, a convenient number if one of the cards in his hand happens to be a Brute Force, which will bring you to exact death.
You should stop the game state and calmly wait for the judge, whilst keeping a close eye on the board to make sure nothing changes. When the judge arrives, you should clearly tell your story of the events, being careful not to leave anything out. It is now standard judging procedure for the judge to then ask your opponent his tale of events; should it differ from yours, feel free to point out the difference, especially if he omitted anything or if his rendition of events brings something else to your mind.
In this case, I would also ask the judge to look at my opponent's hand to note whether there is a Brute Force or not, pointing out the unusual tapping of mana and crucial game state. Normally, in the case of mistapped mana, as you caught the error as soon as possible, your opponent would receive a warning and be asked to re-tap his mana. However, we suspect that our opponent is cheating in this case, and we have called the judge with the intent that he be caught, and not just reprimanded with a warning but hopefully (if he does have the Brute Force in hand) be disqualified and later banned.
My tale would run something like this: "He untapped and drew, and thought for a long period of time. He then incorrectly tapped his mana for the Henchfield and before I had time to respond, swung in with his team. I noticed that his mana was incorrectly tapped and called a judge. I am worried that he tapped his mana wrong intentionally with the intent of winning the game with the Brute Force I suspect he has in his hand, as it is his only way of winning. Would you mind checking to see if this is the case? Thank you." I would then simply wait for the outcome, and if I disagreed with the ruling, I would point it out or would appeal to the head judge.
Tell it to the Judge
In the scenario above, I told the judge everything that happened during the turn in question. Were further information needed—for example, the previous turns' state of affairs if some information can be gleaned from it—I would also have provided it. It is important to keep your story straight and elaborate on every detail you think to be relevant in your first telling. This can include details about what your opponent did with his body, how he held himself, whether you thought he attempted to distract you or try some sleight of hand. You should try and remain as emotionally detached as possible, because nothing will come of you losing your head. If you suspect cheating, mention your suspicion rather than directly accuse your opponent of cheating, for that is for the judges, rather than you, to decide.
Although it is always nicer to assume that people are good, sometimes this is not the case. I think that there are cheats out there, and if your opponent is not cheating, then you have nothing to lose from calling a judge. I pride myself in being able to catch cheaters and I think there are several factors that help me in this respect.
I try and pay avid attention to both the game state and my opponent at all times. If their hand is near their library, I watch it closely so no sleight can be performed. This is especially true whilst my opponent shuffles and cuts either of our decks. I always keep track of how many cards they have in their hand and announce every change of life totals so that no discrepancies can occur. Furthermore, short of being anal, I try and run through the steps and phases of the game as clearly as possible. This is more relevant during the Combat phase than anything else. Above all, I try to keep the game state as clear as possible. It is clear where my lands are, where my graveyard is and what is tapped or not.
One of the most important things to realise when playing tournaments is that the judges are your friends. They are there to help, rather than hinder, you. If you are unsure of the game state, you can ask your opponent, but if he does not know, or is unsure, or you would rather keep what you are asking about secret from him, call a judge. No wrong can come of it; it is one of the reasons they are there.
If you are calling a judge on your opponent, do not feel like you are being evil, even if it is a menial mistake. Often players receive warnings for similar infractions that add up. Failing to take a damage from a painland might seem like an innocuous mistake, but if the player has a history of repeatedly doing it, then it is likely done intentionally and is, therefore, cheating. Don't feel like you have to be a regular tournament player to call a judge on these things; if you are unsure for something and feel like something wrong has occurred, a judge can only help you out.
It should also be noted that most judges, despite knowing the rules better than most players, are not infallible. Some of them do not know everything and some situations are downright tricky. If you feel like a judge is either wrong or overly unsure of his ruling, you may appeal to the head judge. Again, this is something that you should not feel bad about doing; in fact, I think I appeal to the head judge for most of my rulings.
There is also something that most judges and most players fail to do at the end of a ruling: give / ask for extra time. Rulings take time. From the moment you pause the game to wait for a judge to arrive to the end of the ruling, the clock has stopped. Take note of the time the moment you call for a judge and point out how much time the ruling took at the end of the ruling. Nothing negative can come from having extra time.
There are certain types of cheating that you have to be more aware of in Limited. Most of these are due to the complicated combat phase, and the complex interactions of stodgy boards. In Constructed, in comparison, sleight of hand tends to be the most common cheat because there is plenty of card draw and deck manipulation (Sensei's Divining Top springs to mind).
It is your duty as a player to make sure that the game state is correct at all times, not just for your cards and actions, but for your opponent's too. To deliberately not keep the game state correct and up to date is cheating. An easy example would be a new player playing against a Level 6 pro player who plays flawlessly (or as close as possible). The new player has an Essence Warden in play and continually forgets to gain life from it. It is, however, mandatory life gain, and dutifully the pro player continually points this out to his new opponent. If this was a high level competition and the less experienced player continually forgot, it would be the more experienced player's duty to call a judge. The player would likely receive a warning on the first occasion and a game loss if he repeated the offence. This might sound a bit harsh for something positive like life gain (in fact, it might be cautioned on the first offence) but if it were instead, say, the repeated forgetting of a Plague Sliver's ability or a creature with vanishing, the offence seems much less benign.
Also, were the nearly flawless pro to not point out his opponent's life gain, I would suspect he was cheating, simply because he knows better. It might seem harsh having to call a judge on an opponent for forgetting to vanish his creature, but too many players get away with as much as they can because their opponent's are inexperienced or unobservant. It is a good habit to get into, to call every opponent on these minor errors, not just the ones you suspect are cheating. I have been cheated too many times because I did not call my opponent on a slip that seemed innocent earlier in the game, which he repeated in such a way that I definitely felt was cheating later on, and subsequently only received a warning because of my earlier charity / goodwill, instead of the game loss or worse he deserved. This might seem like harsh line to take—in fact many players refuse to take it—but I have heard too many "he cheated me and got away with it" stories to not do this at high level events.
There are several compulsory triggers that you do not need to point out, but you cannot lie about if queried. For example, if you attack with a Herd Gnarr and a Nacatl War-Pride, you do not need to inform your opponent that your Gnarr is now massive beyond belief, but if he asks, you have to tell him how big it is. The same can be said about subtle effects like Watcher Sliver / Tarmogoyf / Crovax, Ascendant Hero. It is not your duty to point out these effects unless they alter the game state (Crovax killing one of your guys, for example).
I've briefly mentioned life totals, but many players find room to cheat here too. This is often easily remedied by announcing every change in totals of both players. However, if you ever find the life totals to be different, you should call a judge. It may be your own fault, in which case it will be you slapped with the warning, but it is more likely that you are not the one in error as it is you calling the judge. Again, with most of these 'little' cheats, it is hard to assess if the first offence is a cheat, but the second almost always is a cheat (or general incompetence, something I do not mind being punished), and the cheats will never get caught if you fail to call them on something the first time around.
One of the most common cheats is from players drawing extra cards. It is often very difficult to stop this happening if you are not paying too much attention to the game state as some cheaters are very good at this, much like the stage magicians that bemuse you even whilst you are trying very hard not to let them fool you. It is arrogant to assume you will be able to always actually catch them cheating, no matter how romantic the idea of catching someone red-handed might seem. Sure, keep an eye on their hands when they go near or touch their deck, but this is a cautionary gesture at best—it is not foolproof.
The only guaranteed way to stop someone from drawing extra cards is to be aware of the game state at all times. Keep asking them how many cards they have in their hand if you are unsure as to what the total should be. If you are ever unsure about the amount they have, don't try and work it out yourself (unless it is really early on), simply note the time, call a judge and go through it with them when they arrive. You lose nothing (assuming you took note of the duration of the ruling so you get the correct amount of extra time) and will only ensure that you are not being cheated. It helps to work this out if you note who mulligans each game, to allow for the card discrepancy.
One of the things I am never sure of is when an opponent presents a forty one card deck. Sure, some bad players do play this amount, but it is often an actual mathematical error rather than an intentional strategic mistake. It puts you in an awkward position; just after you have pile shuffled their deck and realised it has an extra card, there is little you can do other than ask if they are playing that many cards and judge from their reaction whether or not it is intentional. If you suspect their response, feel free to call a judge and ask for a deck check / card total verification. As with most of the things that I am here advocating, this is the harsh side of the law, but if you feel it necessary, do not feel afraid to invoke the judge.
You should always pile shuffle your opponent's deck at the start of the match. Not only to count the cards, but whilst shuffling you should be looking at the backs on his cards. You are looking for worn cards, bendy foils, and marked sleeves. If any cards are obviously different call a judge immediately. This is a very common cheat and there is, once again, no harm in bringing a judge to your aid. At the worst, you'll get extra time, whereas at the other end of the spectrum, you will have caught a cheat in the act.
You should keep an eye on your opponent before the match begins to watch out for mana weaving. This is where the presented deck is (generally) fixed land, spell, spell, land, spell, repeat. This can be done obviously by them sorting the land out blatantly or shuffled into the deck from a pre-rigged deck, so watch out if all they do is one shuffle and then present. If you notice this, rather than shuffle it away, you should call a judge first because what they have done has been to present an insufficiently randomized deck, which is illegal.
One other thing to look out for is when you are playing Sealed Deck. If the pool includes expansion, like Time Spiral-Planar Chaos-Future Sight Sealed, then you cannot have duplicate commons from the main set, or from the expansions if it's the full block. You should call a judge if ever you see a duplicate card in this situation. Sure, Sealed pool irregularities occur, especially if foils are being replaced, but it is always better to be safe than sorry.
When playing against somebody whose main language is not the same as yours, you should always conduct the game in English and as clearly as possible. If there is any understanding make sure it is resolved before action takes place. This is because many people use "language barriers" as an excuse to replay mistakes. This can be overcome by once more keeping the game as clear as possible.
This last piece of advice actually has nothing to do with cheating, but somehow it springs to mind and can win you the game just as easily as stopping a cheat. Take-backs, no matter how slight, will cost you games. You are playing in a tournament, not a friendly game; you win because they make mistakes. This is one reason why you are a better player than they: because you make less. You are not being tight, you are playing competitive Magic.
One final piece of advice—always, always shuffle and cut your opponent's deck at every available opportunity.