Chess engines brings not only knowledge and power to the modern chess. There is also the problem of using computers directly during games to achieve victory.
However, this can truly be called a problem only in relation to serious tournaments. Cheating in online chess is pretty harmless and can be considered more like a prank. But it’s useful to know certain aspects in order to understand when opponent plays fair against you and when he uses the chess engine.
There are two type of chess cheating: "dumb" and "smart".
Most of the cheaters use simple method - start second game on another tab or smartphone against chess engine. They just duplicate original game by copying your moves and translate moves of the engine against you. Take a look:
As you can see, it takes pretty much time for a cheater to make each move. That means:
1. This method does not allow to cheat in short time controls (blitz, bullet). 2. It can be detected pretty easily due to strange delays before each move (especially before obvious moves).
At good chess websites such as lichess.org and chess.com, you can see moves time chart. Flat line on a graph indicates that moves were most probably made with the "dumb" cheating technic.
Also, inexperienced cheaters usually do not bother with the natural (human-like) playstyle. In most cases, they simply use Stockfish - the most popular and very powerful chess engine. Unfortunately for them, this power makes it easier to detect computer moves.
Again, at serious websites you can make post-game analysis, where all mistakes will be shown. If player in every game had 0 inaccuracies, 0 mistakes, 0 blunders and games were longer than 30 moves - most probably this player cheated. Here is analysis of the game above for example. In this case we can see 2 inaccuracies, because old engine Rybka 2.3.2 was used.
There is one more useful advice: check moves of the suspicious player manually with the Stockfish engine. Since "dumb" type of cheaters usually also use Stockfish, then most of the moves will be the same. Take a look at example below - as you can see actual moves match recommendations of the engine almost on 100%.
However, note that similarity of moves in one or two games does not guarantee that your opponent used cheat. It should raise your suspicion, yes. But it is better not to make any public accustitions. Just report this player and let moderators of the chess website do their job.
Summary. To determine if cheating is used against you, follow these three steps:
1. Check moves time. If each move made with the same delay, then +1 to cheat probability.
2. Check for inaccuracies, mistakes, blunders. If game was ideal, then +1 to cheat probability.
3. Compare moves of the game with the engine. If most of them are the same, then +1 to cheat probability.
Experienced cheaters use special software for cheating, for example - chess bots. These programs can make moves automatically almost without delay, which allow player to cheat in blitz, bullet and even in ultrabullet (15 seconds per game).
Some chess bots have special mode, when they just show best move on the screen. It is also pretty fast, so game will look natural. You have very low chances to notice that you're playing against cheater in this case. In this mode your opponent will play the game by itself and use bits of advice from the chessbot only from time to time. Additionally smart cheater can use some old chess engine, which will be difficult to track comparing to Stockfish. All in all, with a proper usage it is very hard to identify cheating with the chess bot.
However, here is advice for you: pay attention to sacrifices and mating sequences. Piece sacrifice is a difficult decision for a chess player. Usually it takes some time to play such move. However, engine can calculate tactic almost immediately. So if your opponent quickly makes "strange", but strong moves, maybe you need to take a closer look to his games.
For example, at the position below (from the game Siegbert Tarrasch vs Romberg) chess bot find brilliant sacrifice in a second. Imagine if your opponent make such move in a blitz game against you with the accurate continuation (it is mate in 11, if you're interested).
So, once again - even if in general a "smart" cheater plays almost indistinguishable from a grandmaster, he still can be caught in trifles, like instantly made brilliant sacrifices or perfectly calculated (with 10 seconds left on the clock in a bullet game) long mating combinations.
Check your games with the chess engine on analysis board, for example at lichess (you can load FEN of the specific position or PGN of the whole game there). We also have our own online calculator here.
It is recommended to enable "multipv" option - this will show multiple lines (variations), suggested by the engine. It's important because smart cheater can play second or third best move in order to decrease overall strength of playing and achieve more human-like style through that.
And don't forget always to check suspicious games for number of mistakes and blunders. If there are zero mistakes in 100 moves game, it is very strange. Take a look at screenshot below: DrNykterstein is Magnus Carlsen. As you can see even champions make mistakes.
Cheating is bad. Cheating in real tournaments is very bad. Cheating in online chess is also bad, however pretty harmless, because nothing at stake (except pride) in casual online chess game. Some ways of cheating are easy to detect, some are not.
However, we can recommend not to make false accustitions, even if you think that you've lost only because your opponent wasn't play fair. First of all, use advices of this manual and check games, which you find questionable. If you have reasonable doubts, then just report this game/player to the moderators. They have much more experience and instruments to determine if any cheat was used.
And in conclusion - don't worry over trifles. Perhaps chess is one of the greatest games in human history. But still it is just a game.
Be healthy, take care of yourself and loved ones. It is the most important.