If you lost, then he cheated. Always.
i was playing a 970 in a rated game first game i won second game he was pretty tough, this is how you know when someone cheats if they become harder than they were in the first round
It can be very hard to tell because Chess is alot about patternrecognition. Maybe you are better than him in 1000 positions and he is better than you in 500 but if you end up in his 500 positions he will win again and again
It's probably better for you to stop resigning positions that aren't lost than presuming someones cheating because they are winning with a lower rating.
A sure fire way to know something is up......when i lose.