How to tell if a man is just using you for sex or if he could become my new potential girlfriend?

Go down on him and shut up!

if he only wants sex

he'll push for sex and get mad if he doesn't get it

try this:

the next time he asks for sex say your tired

if he reacts badly and starts yelling and getting angry yeah he's probably just in it for sex

if he reacts nicely and says its okay he probably just wants you as his girlfriend 

if you refuse sex when he wants it and he leaves you then yes he most definitely in it for sex

Girlfriend?

If  he pushes for sex; bad sign. If you deny him, and he stays; good sign.

Make him wait and if he can't then he only wanted sex from you.