> There are "computer moves" which stand out vs human players. These normally show up in lines where there are many options of roughly equal value and the computer picks a move that is infinitesimally better but out of 'theme' with the position.
To elaborate on this, humans use pattern recognition to identify themes within a position. This is a shortcut that prevents needing to mentally brute-force your way down an enormous tree of possible positions. Elite chess-playing humans are very good at this, but are still very good at spotting potentially non-thematic (perhaps "surprising") moves that offer some quantifiable advantage.
Computers operate very differently from humans. They rigorously evaluate positions to absurd depths. They can examine lines further than 30 moves deep without too much time. Often the moves they come up align with the thematic ideas that humans have (after all, there's a reason why humans have identified these patterns). But at the end of the day, the computer isn't playing thematically. It only cares that the position at the end of best-play by both sides has the score most in its favor.
This leads to computers playing moves that humans would only come up with exceedingly rarely. And if a human came up with that move, there would generally be some clearly-identifiable reward that humans can pick up on several moves later. When a human player plays computer moves, those noticeable rewards are often missing. The cheating human makes a puzzling move, play continues, and many moves later their opponent is worse off. But even after serious analysis it's not entirely clear how that original move brought about this advantageous position.
Of course maybe the human in question really wasn't cheating and stumbled into a brilliancy. Perhaps it's even one they didn't even truly understand the ramifications of when they played it! But when a human makes several of those types of moves in a single game, or even across a single tournament, it brings about extreme suspicion.