The top chess engine was ELO rated at 3546 in 2021. The top rated player ever was Magnus at 2882. To put that ELO difference in context, even if Magnus gets the first move, the chess engine is expected to win 0.979934616 of the time. Within that gap, there are many moves that can be played that are superior to human moves by varying degrees.

It would be very hard to detect a sophisticated cheater solely by examining their moves in a vacuum. They could pick moves that appear to be "human" e.g. moves that appear to be chosen based on the common heuristics that strong human players tend to rely upon, rather than moves based on very deep brute force calculations, where we could never match the strongest chess engines.

The giveaway is usually in the time required for each move. Humans will tend to spend varying amounts of time on each move, with significantly more time spent at critical moments in a game. A computer will pretty much spend the same amount of time for each move. But even here, a sophisticated cheater could disguise this side effect by only using computer assistance at critical moments.

[1] https://wismuth.com/elo/calculator.html#rating1=3546&rating2...