how do you know when your opponent is using a chess engine?!

I think if you after the game analyzes the game on the computer.

and you find out that he always makes a moves that is on the top 3 list of candidate move even after 30 minutes of computer calculation. he is either a GM or someone using an engine.

but I think it is wrong forum to discuss cheating.

If at anytime you feel your opponent may have cheated, you contact support and we check for you.

Take him back to your secret volcano lair and torture him until he confesses.

Very generally, if you don't know how to tell, don't worry about it.

BUT, if you are good enough that you pretty much avoid all tactical blunders, AND you know positional chess well enough to construct, follow, adapt, and execute viable plans...AND your opponent is playing utterly anti-positional junk chess over and over without any recognizable plan...BUT you never gain any ground on him, and in fact fall further and further behind...THEN there is a pretty good chance you're playing an engine.

OR pretty much anytime you lose material to a middle game combination, AND you can't figure out how your opponent saw it, AND when you trace it back, it turns out he had to find a single, practically invisible concrete variation stretching back 8 moves, THEN you are probably playing an engine.

GM's don't match engines nearly that often.

I realize this process would take time, and on the day we contact you it's not as if it will be your first priority, but would you be able to give an estimate for about how long we could expect it to take to check after a ticket is made?

On a good day it can take less than 24 hours for a check to come back, i really can't give any accurate constant timeframe of an answer as it depends on several factors