May the online rating turn out to be a failure?

Hi all:

I have a confusion in my mind. If a guy uses a chess game application while playing chess online, s/he can simply enter his/her counterpart's move to the application then use the computer's move as his/her own move. In this way s/he can win!

Nowadays computer becomes so smart that it's hard to beat. In this sense, how to find out whether a guy is cheating to improve the rating?

Don't be rude if I'm wrong.

time taken to play a move , look for any blunders he/she can make during the game ... moreover the computers play positionally so you'd most likely be able to discern for yourself ... but there is no actual way to find out 

Deeper answer, people using engines on chess.com do not bother me - their rating reflects this so I don't mind playing (say) a 2000 whether or not s/he is a 'real' 2000 or a 1200 'pumped up'. I get the same challenge; I learn the same lessons; and my own ratings changes in the same way.

OTB is a very diffent matter.