How Do Casinos Find Card Counters?

Even though card counting is legal (if you only use your brain), it is frowned upon by casinos; therefore, they will use several different ways to catch suspected counters.

Methods for Catching a Card Counter

1. Pit supervisors and/or surveillance personnel will watch a player’s bet spread because they know that card counters have to vary their bets to get the edge. 
2. They will watch a player’s demeanor, specifically:
   1. is he always serious?
   2. is he moving his lips?
   3. is he talking to other players and/or the dealer?
   4. does he show any emotion when he loses a big bet?
   5. does he tip the dealer?
   6. does he order an alcoholic drink?, and
   7. is he paying close attention to the other players’ blackjack hands or staring at the discard tray, to ascertain how many decks have been played? 
3. They will watch how much a player bets on the first round after a shuffle. Card counters usually will bet the minimum because they do not have the edge at the start of a new round after the dealer shuffles the cards.
4. They will watch how much a player bets on subsequent rounds after the shuffle. Card counters will start increasing their bets once there is an imbalance of high- vs. low-value cards in the remaining unplayed cards, and this usually occurs after several rounds of cards have been dealt.
5. They will take note of how much a player bets after a push (or tie). Most gamblers will leave the same bet amount after a push; however, card counters often will change their bet depending on the current status of their count.
6. They will watch how a player plays his hard 16 (e.g., 10 + 6) when the dealer has a 10 upcard. If the player has a large bet and always stands, but with a small bet always hits, this is an indication that the player may be card counting.  (When a counter bets large, it’s because large cards are abundant in the unplayed decks of cards. When faced with a hard 16 vs. dealer 10, he will stand to avoid busting. On the other hand, if the counter bets small, it’s because the unplayed cards are richer in small cards; thus he will hit his 16.)
7. They will watch how much a player has bet on his hand when he takes insurance. (Card counters only take insurance when they have made a big bet on a hand because the count was positive indicating a greater chance that the dealer will have a ten downcard and a blackjack.)
8. They will shuffle the cards after the player makes a large bet to observe the player’s reaction.
9. Some casinos use facial-recognition software to determine if a player is a known card counter.
10. Some casinos hire a third-party consultant who can analyze and confirm that the player is a skillful card counter with either a live feed or a video of the player playing blackjack.
11. Most casinos have a database of known card counters. They will get a photo of the suspected card counter and compare it to their database.
12. Some casinos will dispatch a counter-catcher to observe a player. (Although this was done fairly frequently years ago, it’s not so common nowadays.)
13. Often a casino will review a video of the suspected card counter after he leaves,  and if it’s determined that he was counting, he will be barred or trespassed the next time he tries to play. 
14. Some casinos use counter-catcher software to input every card played after the shuffle. The software will determine if a player is card counting.

Conclusion

Besides being on the lookout for solo card counters, casinos will also be on the lookout for card-counting teams because well-financed teams can wreak havoc on a casino’s bottom line. Casino surveillance personnel or floor supervisors will look for players who appear to be signaling other players who then join a table making large bets and then leave. A casino will often check with another casino if the same group of players has played there together. Casinos will often check their database of known card counters to determine if any of the suspected team members match.

Some of the newer technology that is available to casinos to catch card counters includes dealing shoes that scan all the cards being dealt and special intelligent casino chips that track player bets automatically. (I haven’t heard, or read, about a card counter being caught with this available technology, yet.)