He uses a wide opening repertoire
Every game, the computer chooses a random opening. If you challenge your opponent game after game, you will see that he is a great theoretician. A chess engine won’t wait for the middle of the game to take an advantage; it will outplay you right in the beginning. The time of thinking is short because the engine doesn’t calculate until it is out of the book.
He takes the same amount of time to play a move
When there is contact between pieces, we normally enter the calculation stage which is time-consuming. Sometimes there is a position when the move is evident or simply we should play intuitively; in this case, we don’t take much time. The chess engine doesn’t respect this pattern. It literally uses the same amount of time for every type of position.
Its statics is weird
You can easily find a cheater by looking at his statics. If we see an abnormal curve like a drop of 150 points in a couple of days then there is something wrong. Fortunately, the majority of sites give you access to the history of your opponent. You can investigate your opponent’s progression and draw a conclusion about the use of an engine.
His moves are tactically flawless
A chess engine calculates very accurately, even in the mega-complex position. At this point, the chess engine is far beyond human capabilities. How can you be sure that your opponent precisely plays every move? It’s very simple; just analyze the game with a chess engine. It will evaluate every move. If you find a perfect play you can ask your opponent which chess engine he is currently using.