The basic measure is “accuracy”: the percentage of moves that are the best move for that position. Computers are, as you would expect, inhumanly good on this measure. Better than the best human players, who will have “inaccuracies” even in their best-played games. It makes spotting cheaters pretty reliable and easy. Niemann has been busted for cheating online repeatedly, and analysis of his in-person play bears the same hallmarks.