Question about cheating with engine

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2700orbust

What is the giveaway that someone is cheating - is it the final accuracy % or best move totals and lack of errors and blunders.l know they are related but what is the most important ? l saw an 1100 beat a 1650 - the depth of the moves would make Stockfish envious. l know upsets happen all the time but not with his final numbers  .Can someone explain ?

Jack3574
Yes he was cheating