theory11 — Magic Tricks & the World's Finest Playing Cards
Normally I would ignore this type of thread but jrobarts05 provided incorrect information which I feel I should correct for future readers.
Counting cards does not "increase the number of victories." Counting in BJ is based on betting more when the cards are in your favor, and less when they are less desirable. It's not the number of wins that changes, it is the amount of money won and loss during these set amount of wins and losses that changes.
The biggest tell of an individual counting cards is the fluctuation of their bet (more when high cards come out, less when spot cards come out), since this is the very premise of counting cards. Another tell is the player trying to read cards from across the table, or staring at other player's cards. Spotting crews are much trickier.
-Nicholas
PS: Trying to spot a counter by "looking for signals" as suggested by jrobarts05 is a bit like a hypochondriac with "Google-itis." Everything will look like a signal.