It's the way he moves, it's the way he talks, it's if he wants to satisfy you as much as he wants to satisfy himself. If he's quite happy to do the deed and then leave you to your own devices, you can be pretty sure that's just sex. Equally if he has no time to help you get in the mood, he just wants to do it right now even though you're not ready, that's probably just sex.
Alternatively, if he wants to take the time to make sure you enjoy yourself to, if he sets up a candle lit dinner, gives you a backrub, plays around a little before hand, then takes care of you while you're having intercourse, by which I mean he talks to you and listens to you and knows what you like and does it, then you're in luck and you've found someone who loves you. It's a good sign if he sticks around afterwards for a cuddle too!
That's my feeling on the matter anywyay.