This. I've talked to several people who work in casino surveillance. They all said that if they think you're counting, the very first thing they'll do is assign an agent (or maybe a computer, these days) to count along with you.
Make an error or two (vs. perfect counting strategy), and they'll let you keep going, because long-term they'll still have an edge. Count/play perfectly and you'll get backed off or 86'ed.
They other key tip off to get someone assigned to you is your betting patterns.
It's well known that when you're counting cards, there is always a swing to when the player has an advantage. If you're upping your bets when you have the advantage, and then lowering them when the house has the advantage, they'll usually stop you from playing at the blackjack tables and either ask you to leave, or tell you to play something else where they have the advantage, like slots.
Not all casinos, and not even all tables within the same casino. Many of the Vegas casinos will have infinite shuffle machines on the lower-value tables ($5/10 per hand minimum) but have standard 6- or 8-shoe decks on the higher-value tables. The other trend lately is to make the rules less favorable on cheap tables (H17, no resplit, no DAS, etc.) and force you to go to a $25, $50, or even $100/hand table to get the player-favorable rules.
say you're playing a $5 game and your bankroll is $100. If you make one mistake, either at counting or basic strategy, you just gave up 5% of your ROI, over 3x the "edge" you get by counting. I remember hearing the slogan "If you can't play absolutely perfect basic strategy for hours at a time without making a single mistake, counting is not for you".
Which prompts the question of whether anyone starts off with an error or two to 'beat' this check.
Obviously errors aren't sustainable, since the edge is so small, but I wonder if it might be possible to open with an error so you can stick around a little longer before someone checks back in and bars you. More broadly - are counters these days trying not to get noticed, or trying not to get banned?
Once you start winning a good bit of money, the casino is going to be very aware of you. It's their business to know their customers, whether they are cheats or high rollers. If you show up with not a lot of money but win a lot, they are going to watch you. You may not automatically be dinged as a cheater because you give the house their cut every now and then but they know what you are doing. It's only until the point where you begin to eat into their profits that you will be shown the door. The casino knows how much money they need to pull in a night to be successful. This is why big spenders get free stuff.
It really depends on what type of game you are trying to cheat if you are avoiding detection or avoiding a ban. With slot machines, if they think you are up to any mischief, you will face legal issues. As for cards, the casino is going to be aware of your activities if they have any inkling of things like card counting. The dealers know what to look for and you will be watched. The casino is just trying to make sure you don't take too much from them but they also want you to keep winning, it makes others around you think they have a chance too. Make a few mistakes here and there and you delay the moment when they tell you to get lost for the night. You will get noticed so your only option is to just avoid getting banned.
You want to be a bit careful with your terminology: messing with a slot machine is cheating and will get you arrested, counting cards is NOT cheating nor illegal, but will get you backed off or trespassed if you're good at it.
Make an error or two (vs. perfect counting strategy), and they'll let you keep going, because long-term they'll still have an edge. Count/play perfectly and you'll get backed off or 86'ed.