Underground gambling or casinos are a playground for accomplished cheats. They probably have been busted in regulated casinos or at the very least have a lot of steam. Underground is somewhat of a "safe-haven" for them. There are so many angles they could be shooting, several of which have been mentioned in this thread.
Roulette past posters, "Savannah Players" (reverse past-post), are some of the most difficult to detect, due to their incredible skill. I have had to slow mo video tape and watch it several times to pick up a really good roulette move.
There are far more cheats that mark cards than any other scam. One of the toughest to detect is a very tiny razor which is super glued to an index fingernail. The marker will then make minute almost undetectable marks, usually in the center, of the back of the cards.
Another remarkably undetectable marking scam uses an ink that can be seen through a specially made contact lense. The thing that differentiates this "invisible-ink scam" from others is that the ink used completely dissappears after about 45 minutes. This gives the team ample time to make a nice blackjack haul and leave no evidence.
The world of scamsters is densely populated with gifted single-0 hold out artistes as well. Of course frequent deck changing and counting can stop these thieves in their tracks.
The best defense is to simply bar or max bet a suspected cheat. Anyone taking frequent wins from a game that has a 1% house edge should be heavily scrutinized.
