Ways in Which Casinos Know that You Might Be Card Counting
Casinos have a lot of tools against card counting. In the end, you might get banned from the casino, or banned from table games.
What the casino might do to bar you
After each shift, the dealer will approach the pit boss and tell him/her about card counting suspicions. This is why we suggest, when the dealer leaves, look for where he goes and if he is pointing to your direction. The pit boss will start observing the play by walking around. If he sees a substantial bet spreads, he might look through the discard tray and count cards backwards to check if your play correlates with the cards that have come out. He might also simply walk up to you and start talking to you in a friendly manner to make you lose your concentration. This is why some people do not start out with 1 unit bets. They might switch between 1-3 unit bets. Moreover, this is why counters train their skills in a way that counting becomes second nature, so that when they do get disturbed, they can continue counting.
When the pit boss starts suspecting something, he might use the phone to call the security for a skills check. They turn the surveillance cameras on you and may analyze your previous play to see if you are in the system. If you are in the system, the pit boss might mark you as a counter in his or her notes. The security of the casino is basically retired card counters so they can tell quickly who is counting and who is not. Then they assess if you are a threat or not. If they think you are a threat to the casino, they come down with a group of people and tell you that you are too good for the game.
Then they might share your information and pictures (collected from surveillance footage) with other casinos. Then you will find out you are banned from other casinos as well.
What the casino might do to prevent people from counting
1. Make the rules worse. Remove doubling after split. Remove late surrender. There are several things that they can do to increase the house edge. Regular players know that bad rules equal fewer winnings too, so many of them would be less likely to play.
2. Use automatic shuffling machines. This makes counting useless. Most players and sometimes even dealers hate these machines.