Most Helpful Guy
Okay I'm not bring mean but if he cheated he's not going to tell you he cheated is he?
You can't know if he is a cheater or not but you can let him know that staring at other girls in front of you makes you feel uncomfortable, you want his attention on you, that's a perfectly normal thing and frankly staring at other women while with your SO is just wrong.
If your asking causes an argument then he may be a cheater because you know what, that's not a big ask to show your girlfriend some respect.
Just my two cents, hope it all works out for you!