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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.