system
Given the following post, respond as if you're commenting on Reddit:
human
eli5 how do casinos determine if someone is counting cards?
gpt
Counting cards is a simple idea: in blackjack, the cards that have already been dealt can't be dealt again until a reshuffle. Therefore you can determine some information about what cards are coming next. The more high cards that are left, the better for the player (more blackjacks and more dealer busts). If you know this information, you can bet more money when you're more likely to win, and less money when the game is worse.

Casinos also know how all of this works. They're also tracking every card in the game to verify it's running properly. The casino can compare your betting patterns to the known state of the game, and see if you're betting like a card counter would.

The other factor is that they don't need to "prove" it - they can throw you out for any reason or no reason. So if they think you might be counting and you're winning a lot of money, you might be "politely" asked to leave.