First off, if you are playing against a true "mechanic", you're doomed. True card handlers' illegal movements are invisible to any but the trained eye, and you need to have been a part of this craft before you can identify it. Many kitchen table gamblers sometimes choose to falsify a deck of cards because of the glorified image of a "mechanic". They can deceive anyone who isn't looking and that makes them feel like a master trickster. Mastering the art of card handling is like a full-time job that has a training period of decades, though. That devotion is not something that appeals to everyone. Most cheaters depend upon some very, very basic moves that are easy to spot. Below are some basic things to look out for...
This involves keep a card in your sleeve, on your lap, or anywhere that it isn't visible to other players. The card is acquired from a previous game by throwing one card directly into the muck and keeping the other one. This gives the cheating player an extra card to swap with either of his existing pocket cards. The best defense is to watch a player who keeps his pocket cards under the table, and count the deck in between games.
False Shuffles
If one knows the top or bottom card of the deck, it's not too difficult to shuffle a deck so that either or both of those cards stays at the top or bottom, while mixing up the cards in between. The easiest two types of shuffles that can be used to set the top/bottom card are the riffle shuffle (or dovetail shuffle) and the overhand shuffle. Cheaters will use these shuffles in combination, along with some false dealing to give any player certain cards. A simple way to prevent it is to make sure that another party cuts the deck before the deal. This will still give the dealer only the slight advantage of approximately knowing where the card(s) are in the deck, but this usually isn't a large enough advantage to amount to anything. After the set deck is cut, the dealer also has the advantage of knowing that the former top card of the deck is directly beneath the former bottom card after the cut. That's why we have burn cards.
Riffle Shuffle
A riffle shuffle is when one cuts a deck in twain and flutters the two sections together by applying pressure to the back of the cards while pulling up on the front with your thumb. The thumbs simultaneously release the pressure to make the cards flutter together. Then the two sections are pushed together. The way to cheat, to put it simply (because it is), is to note which half of the deck was the bottom half and which was the top half after you cut it. A handler will make sure the bottom half releases first, putting the bottom card on the bottom. One can also make the bottom half stop fluttering before the top half to make sure the top card stays on top.
Overhand Shuffle
This is the most common type of shuffling. It is when the shuffler holds the entire deck in one hand and throws or pulls part of the top of the deck into the other hand. This is done several times till the deck is completely in the opposite hand. The way to cheat involves a "transfer". In this case, the cheater will be putting the top card on the bottom of the deck, or vice versa, or both. With the first motion, they will pull away only the top card, and make sure that the rest of the cards fall on top of that card, putting the top card on the bottom. With the last few pulls, they will make sure to minimize the cards in a manner so that only the bottom card of the deck remains in the original hand. Then they'll simply throw that card (previously the bottom card) on top of the deck (now the top card).
False Deals
This is a deal where a card besides the top card is dealt. It takes a lot of practice to make it look clean and convincing, though. By practicing these deals yourself for a while, you'll become familiar with the difficulties of doing them. This will make a false deal stand out when you watch a potential cheater deal cards. We'll discuss two types, the bottom deal and the second deal.
Bottom Deal
This is the most prized trick of a gambler. It is simply dealing the bottom card of a deck instead of the top one. The term "Mechanic's Grip" refers to the particular way a deck is held when performing the bottom deal. That's when the left hand holds the left side of the deck between the heel of the hand and the tip of the second finger. The left thumb pushes out the top card in an attempt to disguise the trick deal, while the right hand makes the motions to toss out the bottom card. There are two good indicators that a card handler might be dealing from the bottom. One is that some bottom dealers use a swaying vertical motion to try and cover up the fake deal. The other is that the act of releasing the bottom card can give off a particular snapping sound. If you see or hear one of those indicators, start watching the dealer very closely.
Second Deal
This is usually used when the dealer wants to give himself the top card, so he deals the second card in the deck out to everyone else so as to not lose it. Since this requires the dealer to pull off several second deals in a row, it is not as desirable as the bottom deal. There are several ways to do it, but all of them use a telltale rocking motion with both hands to help disguise the act. Watch for it.
All of the above things are pretty basic. In fact, the false shuffles described above are so easy that even an untrained card handler could pull them off if nobody was really watching (check out this false shuffling video for a video of a non-professional doing exactly that). Always keep in mind the following rules of defense against cheating...
1. Watch the dealer habitually, just as you should be ritualistically watching players when they look at their hole cards. Make sure to watch the hands more than the cards.
2. Anyone who can do card tricks has the potential to do them in a game while dealing. If a card handler shows off some fancy tricks with the deck at any point (which they usually can't resist), then look out!
3. If a player consistently gets great hands when they deal, alarm bells should be going off in your head. Don't label a player with good luck as a cheater immediately though. Watch them. I can't say that enough