I can't find a citation but I recall that computers agreed with the moves of grandmasters from the pre-computer era about 60% of the time.
Generally, the biggest heuristic for identifying cheating is identifying somebodies moves share statistical similarity to the top moves of common engines (Especially stockfish). This doesn't really work after a single game, but anybody playing the top move of stockfish 90% of the time over 100 games is a cheater. Nobody that isn't cheating can do that. Cheaters are savvy though, they will notice in a position there are maybe 5 decent moves they can choose from, so for just that position they will choose stockfish's 5th choice. Or maybe they'll only check the engine at the most critical moments of the game and turn the engine off and play normally afterwards. Notice that the person being debated in this article is somebody with a history of cheating, the evidence they cheated in this specific game is likely not as good as the evidence they are just generally a cheater.
On top of this more empirical analysis, there's more subjective analysis. Humans tend to try to simplify games when they're ahead to reduce computational complexity, but computers don't do this as it's not a good strategy for a computer. Humans will tend to follow a narrative and follow a general idea throughout a game with ideas they calculated earlier in the game or in their preparation, whereas computers don't care about narratives and will completely switch plans on a dime. In the endgame the computer starts having a LOT of winning moves that it hasn't calculated to the end and can start making very offbeat choices, whereas humans tend to use a set of rote memorised strategies that are known wins. Again though, a skilled cheater realises all this and will choose weaker more human-like moves that are probably the engines 2nd or 3rd choice.
There's also metadata. Cheaters usually take a few seconds to think about a move that a human would make instantly (this came up in the article where it took the cheater 20 seconds to make their first move), they probably exhibit different browser/app interaction habits. Humans have all sorts of particularities about UI interaction and time management. A lot of people play blitz and bullet chess because cheaters struggle to cheat convincingly under time pressure.
At this level, players know all opening lines to some depth. The first move? I don't see how 20 seconds indicate anything other than a passage of time.
Well in this case it did actually indicate something other than the passage of time, it indicates the player was using a shoe operated cheating device an remarked on how he would normally move more quickly.
Maybe I overemphasized the first move because it was emphasized in the article, in general cheaters use their clock in weird ways throughout the game since for them pretty much any move is similarly complex. In this case the shoe operating cheating device raised the minimum time to make a move, which is a characteristic common to many forms of cheating.
Generally, the biggest heuristic for identifying cheating is identifying somebodies moves share statistical similarity to the top moves of common engines (Especially stockfish). This doesn't really work after a single game, but anybody playing the top move of stockfish 90% of the time over 100 games is a cheater. Nobody that isn't cheating can do that. Cheaters are savvy though, they will notice in a position there are maybe 5 decent moves they can choose from, so for just that position they will choose stockfish's 5th choice. Or maybe they'll only check the engine at the most critical moments of the game and turn the engine off and play normally afterwards. Notice that the person being debated in this article is somebody with a history of cheating, the evidence they cheated in this specific game is likely not as good as the evidence they are just generally a cheater.
On top of this more empirical analysis, there's more subjective analysis. Humans tend to try to simplify games when they're ahead to reduce computational complexity, but computers don't do this as it's not a good strategy for a computer. Humans will tend to follow a narrative and follow a general idea throughout a game with ideas they calculated earlier in the game or in their preparation, whereas computers don't care about narratives and will completely switch plans on a dime. In the endgame the computer starts having a LOT of winning moves that it hasn't calculated to the end and can start making very offbeat choices, whereas humans tend to use a set of rote memorised strategies that are known wins. Again though, a skilled cheater realises all this and will choose weaker more human-like moves that are probably the engines 2nd or 3rd choice.
There's also metadata. Cheaters usually take a few seconds to think about a move that a human would make instantly (this came up in the article where it took the cheater 20 seconds to make their first move), they probably exhibit different browser/app interaction habits. Humans have all sorts of particularities about UI interaction and time management. A lot of people play blitz and bullet chess because cheaters struggle to cheat convincingly under time pressure.