You can't know for sure.
If they make their moves at regular intervals of 5-10 seconds for both easy recaptures and difficult moves then I get suspicious. After that if I review the game (and it's blitz) and I can't find a single improvement for them then I get really suspicious and may check the game with an engine.
This is really rare in my experience though... and that's only for people who cheat every move. I hear chess.com has some fancy auto detect stuff that checks live players though, so I wouldn't worry about it. And frankly, unless you're a really strong player, a cheater is going to mate you in 20 some moves... if it's an even game until you slip up then it's safe to assume you weren't playing a computer.
And yes people will leave those kinds of messages after an upsetting loss, they don't mean anything.
Just wondering as I have seen on a few profiles people have posted notes along the lines of " this *&#% is a dirty cheat" or "uses engine to cheat do not play" etc............
So how do you tell, how can you definitively state a person is a cheat?.... maybe they're just better then you?...
I don't know if I could make a comment like that, without proper proof I would consider that defamation or just spiteful jealousy...
So can any longtimers here tell me how to tell if someone is a cheat? More out of curiosity then anything else as I have beaten a few people with these sorts of notes on their profiles and I know I wasn't cheating so it makes me wonder if the comment was posted maliciously.