text: how can i tell that my girl is cheating on me.? what behavioural changes should i look out for? if your grl i s acting different then she used to, then she might be, but that doesn't mean she is. you might want to look for changes like, she doesn't call you as often as she used to, (if you are having sex) if she doesn't "feel" like it. it just depends on how long you guys have been dating. you shouldn't jump to conclusions if your not sure.