There's an interesting article on how security detect card counting[0]. My take is that it's not a science, instead the security/ pit boss' who spend significant time watching players would develop a 'feeling'.
It is actually quite easy to spot counters depending on how greedy they are. In order to successfully count you have to bet proportional to how positive or negative the count is in your favor. It is not uncommon for someone that knows nothing about counting to vary their bet a bit based on how much they are winning or how "lucky" they feel, but if you see someone varying their bet widely (Say min bet -> 32x min bet) then all one has to do is keep a count as well and that person is easily detected. Many dealers and security personnel keep counts anyway.
Casinos will use continuous shufflers which kill counting 100% or will cut off multiple decks at the end of the shoe so one cannot get a very positive count and more certainty that the deck is in their favor. In addition, as stated earlier, the type of counting these people did in the early days which was most lucrative is easily spotted. So now modern counters have to apply "camouflage" which includes placing higher bets when the deck is not in their favor to make it appear they are betting randomly, and also reducing their bet spreads. All of these eat into the theoretical return, and make it much less lucrative if not entirely not worth it. So modern advantage players look for casinos that don't watch as carefully or dealers that are not cutting off enough cards.
They likely do some kind of cop tricks to get the player to admit to counting cards. I wonder how many people have been banned from casinos for bragging on social media about their skills.
They don't need to get the player to admit it. It isn't illegal, and is easily spotted in many cases. Casinos are private establishments and can either toss the player out, or if they are nice, allow them to only make the same bet every time (flat betting) which makes it impossible to make money counting. Egregious counting which makes decent returns will be spotted almost immediately anywhere that isn't asleep. More subtle camouflaged counting is harder to spot, but the return is also much less. So whenever that player starts varying their bet more, or moving to high roller tables they will get more scrutiny.
I think you misunderstand me. If they get the player to admit he can count cards they can just ban them and justify why they did it. The casinos even share info with each other. There are talented people who probably brag after winning a lot and get banned forever.
To make it even weirder you can self-ban yourself statewide from casinos in most states that allow gambling. It is actually a crime for you to go back in[1]. Not sure what the punishment is.
Oh, you want to know the punishments for willingly breaking responsible gambling rules?
Huge fines, potentially followed by loss of license.
You do not want to go there. With the elevated scrutiny on gambling operators (triggered in part by the US opening up, in part by the increased competition, and in part by the receding margins) all the regulators are itching to make examples out of suitable villains.
Disclosure: I work for a [UK] gambling company and deal with compliance matters on an almost daily basis.
Worse; they're subject to the public's opinion. A player says "They're throwing me out because I won too much! Go someplace else! These guys are bad sports!" and they may lose business. They have to have something obvious and convincing on a person, especially if that person is well-known.
Casinos have way better information on large players' win/loss figures from in-casino surveillance (eye in the sky and pit bosses) than they could ever get from social media.
Casinos will use continuous shufflers which kill counting 100% or will cut off multiple decks at the end of the shoe so one cannot get a very positive count and more certainty that the deck is in their favor. In addition, as stated earlier, the type of counting these people did in the early days which was most lucrative is easily spotted. So now modern counters have to apply "camouflage" which includes placing higher bets when the deck is not in their favor to make it appear they are betting randomly, and also reducing their bet spreads. All of these eat into the theoretical return, and make it much less lucrative if not entirely not worth it. So modern advantage players look for casinos that don't watch as carefully or dealers that are not cutting off enough cards.