Nicholas S is mostly correct on all points here, but I'd like to add one thing. The question is, "how can you tell if someone is counting cards?" The answer, barring certain circumstances, is, "you can't."
Assuming that the question is coming from someone that isn't an advantage player or a game protection expert, the answer is that you simply won't be able to conclusively tell apart the actions of a card counter versus the actions of a civilian that varies their bets and makes strange plays based on hunches. Card counters will increase their bet incrementally when the remaining deck composition favors them, and they'll make deviations from "proper" basic strategy when the count warrants, but these plays are mostly indistinguishable from the actions of a civilians unless you're also knowledgeable of card counting techniques. Sure enough, civilian players are ejected from casinos all the time, especially in Vegas, simply because they make large bet variations and seem to be winning. Pit bosses are quick to act in cases like this, and they often make inaccurate assessments. Casino surveillance folks actually use analytical software to determine if a player is counting or not.
Even amateur card counters find themselves suspecting players at their table of counting cards based on bet variations and strange but appropriate plays, when in fact this is usually just coincidence. It's very unusual to run into a card counter at your table, but new counters seem to recognize the usual telltale signs much more frequently than the odds would suggest.
An old adage: a counter asks a card mechanic how he can spot a cheating dealer. The mechanic asks if the counter is a cheater, to which the counter responds that no, he isn't. Then he can't spot a cheater, the mechanic replies.