Interesting question. They actually employ former card counters and have them sit in the security booths and watch players via the security cameras. If they see a lone card counter raising and lowering his bets, they either send more drinks to slow him down, or they’ll eventually send a pit boss or security to ask him to leave. Catching a team card counter is slightly harder to do just through the “eyes in the sky.”
To catch card-counting teams, they usually hire undercover private eyes or detectives dressed in plain clothes who roam the casino floor and look for the said behaviors that team players can use to signal other team members.
Some MIT card counters actually got mad when Bringing Down the House came out because they said that it was “outing” their methods of disguising signaling card-counting.
The reality is that it was only a matter of time before casinos caught wind of what these techniques were.Source: slate.com