Watch the players' playing styles. Develop an awareness for agitated or nervous people. Cheats and counters very often broadcast their feeling of guilt. Watch for the player's eyes. Be suspicious of overly friendly players.
Counters are sometimes too friendly and overdoing their act.
Try to distract a counter during a plus count. Watch a player who increases or decreases the bets or who varies bets considerably. See if the players go to a bar or restaurant after appearing not to know each other at the table. Watch for players who come up to the game in progress and make a big bet. They may have been back counting or called into the game by a confederate.