The most obvious way they are caught is changing their bets.

If you bet 10$ every hand, then suddenly 100$ when you think the count is good, and are winning, then back to 10$, you're done.

Good card counter will level their perceived variance by dumping money back, just like a poker player will sometimes call with a wider hand range than they consider their play style to be profitable in.

One of the ways casinos combat card counters is to lock their betting. So, they'll see someone with variant betting, between 1 and 100, and tell them they can keep playing, but only if they bet 20$ per hand.

That's an easier thing to kill card counting than outright banning players, as they still may be losing players at slots or some other table game.