system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can casinos tell if you're counting cards playing blackjack?

Unless you were actively illegally cheating aren't they just banning players based on a hunch?
gpt
There are certain red flags that can alert them, including:

- Players watching the cards more intently than others;  

- Players sitting at "third base" (the last hand to be dealt to - a preferred spot because you can see more of the cards before making a decision);

- Players watching what other cards come out before putting out a split or double wager;  

- Bets varying up and down dramatically (this is the most significant thing); and  

- Deviations from basic strategy.

Some pit bosses also have a rudimentary knowledge of card counting, so if they start counting a shoe themselves, and see a player raise and lower their bets at the "right" times, this too will be a red flag (admittedly they won't know what exact system you're using, but all systems generally have you increase your bets when the conditions are to the player's advantage).

Casinos also have cameras pointing at *everything*, so someone, somewhere can be remotely watching even *more* closely to determine if the player is probably counting.  

Having said all that, it's not like they have to establish proof beyond a reasonable doubt; it's a private company so they can just tell you they don't want your business if they believe you're counting cards.

EDIT:  You referred to "cheating" in your post (perhaps inadvertently); for anyone wondering, card counting isn't cheating by any reasonable definition.  You're accessing the same information all the other players have, and you're making betting and playing decisions that you're allowed to make.  The only thing you're doing differently is mentally keeping track of that publicly available information, and using it to make certain decisions.  It all happens inside your head.  That's not cheating.  It's thinking.