It is said that modern chess sites can indentify when someone is playing with the help of a bot or mirroring plays from a mirror match in another site, but how? What does the computer do that professional chess players can’t do?
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A computer can evaluate how good a move is.
In a very raw way: There’s a complex function (we will call it G(b) ) that gives an grade to each possible state of the board (‘b’) . It take in account the pieces that are on board, the position of the pieces, etc. If the value is near 0, it means the game is balanced, if the value is above 0, it means white has advantage, and below 0 mean black has the advantage.
So, the way that computer decides the best move is something like this: Let’s say in the middle of the game, the next to move is the white player. The computer will check all possible moves, and see which one translates to a higher G(b)… BUT WAIT… Each move that white does, gives a different scenario for black, that needs to be evaluated too, so the computer needs to check which move is the best, taking in account the best response of black, and the best response of black for each move needs to take in account which is the best response of white, and it goes on…
It suffices to say that the problem grows really quick. There are some optimizations for this search, but we don’t have computers that can actually “solve” the game. However, estimating the best move by looking 20+ moves ahead is far better than what any human can do.
So, how do we check for cheaters? The idea is giving a grade for each move based on G(b) after a finite amount of moves, and we compare the grade of each move done by the player, with all the grades of the other moves that the player could’ve done, and then you can assign a probability of that being an engine. In other words: If the site sees an unknown person having a performance over multiple games that is far superior than the first rank of FIDE (Magnus Carlsen), and some games in between playing poorer than an average chess players, that’s really suspicious.
Also, we can use the time for each move. That’s a bit more complex, because it isn’t only about checking if someone is taking the same time to do every move (that’s the easy case). Some players will only use help from engine in certain moments, the site needs to identify when this happens. I don’t know exactly the method of each site, but it would be something like comparing the time that player takes to make a move with a database they have, and seeing if there’s anything odd.
Also, I’d like to add that probably machine learning algorithms may have a play in catching cheaters, so there are probably patterns that not even who designed the anti-engine system may be aware of.
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