I once heard if you screw up as a magician, you're laughed at. But screw up as a card cheater, you'll get a fast five in the snot box. The techniques of a card cheater need to be better because there's more at stake! The deduction is plain - if you want to excel in card magic, study the grift...

So how does one catch a cheater in the act? The answer is simple - don't watch. Any cheater worth his salt wouldn't make a move while you burn his hands (watching closely). Fortunately, most aspiring cheaters don't have the nerve, so their technique suffers. Not that I know from experience, but card cheating is 10% technique, 90% confidence. They build relationships with their marks, developing a charming and relaxed air that--if the rapport is strong--will be matched. And it's in that moment of relaxation, when wool is pulled over your eyes. In the off chance you're in the room with an expert card mechanic, watching him is the worst way to catch him in the act. He has to feel that you're none the wiser, and just as susceptible as every other mark he's cleaned out.  

In this clip, Paul Newman gets the point across astonishingly well. He appears to be slightly inebriated and aloof when he sees his mark ring in a cold deck (Bring in a stacked deck. It's called a "cold deck" because it's been sitting, untouched, waiting to be switched in). You can watch the mark prepare his cold deck at 40 seconds, and make the switch at 45 seconds. But the mark doesn't realize who he's trying to cheat. The best. The Carrie Watson* you might say.

*The Carrie Watson. The Best; anything or anyone of high quality or high attainments. From the old Carrie Watson House in Chicago. (The Big Con: The Story of the Confidence Man by David W. Maurer)

P.S If you're at someone else's game, and you spot a cheater, it's best to come up with an excuse and go home. There's no way to tell who is involved and whether or not they're dangerous. The rule being, dont play with people you don't know. 

P.P.S The best way to avoid being cheated is quite simple. Don't play.