How do I find out if hes cheating on me?

Ok so I have a boyfriend who wants to get married to me. So about a month i didnt talk to him on myspace cuz i wuz busy. So wen i wuz able to get on i saw a comment frum a girl dat said hecka stuff. so i im my boyfriend n said are you messing wit sum one who is dat gurl who wrote u da comments he said dont worry bout her sje long gone. n then wen i went back on his page she wuz gone frum his page. how do i find out if he cheatin talkin to another gurl. i mean he asked me to marry him n i dont kno if he messin wit hur or she wrote it cuz she like him n she is jealous of us wut do i do