How can you tell if a guy is only coming back for sex?

If he's not going out of his way to do the nice little things that a boyfriend does, then it has to be pretty obvious at that point. Not that he would admit it, though.

If you ask him what you like about me and he says "Your personality, and because you are hot."

if he doesn't want to meet your parents, he is probably just enjoying the ride...

If he does not take you out on dates, or spend time with you without fooling around all the time.

If he only calls you late at night, he probably just wants sex.

If he gets mad if you don't give him none, cheats, threatens to break up with you...

If he can't just spend time with you without having sex. He'll want to know your feelings, interests... etc.