How do you know he's really interested in the realtionship and not just sex?

If he takes things slowly and doesn't rush you at anything. He asks you questions and seems genuinely interested. Respects you. Come on it's easy to spot a player even though girls are attracted to the players though...shame

ANSWER:

You can tell if all he wants is sex from you. I guess I will say just use common sense and if you can all the sign that he do or did will be clear. When a man becomes close to you and you can really tell because he will make you feel special and you will be the center of his attention. A man will be happy just to be with you. He rather talk and get to know you in a lot of ways. He will be interested on where you came from, your dreams, your life schedule and many more. Don't get me wrong he will be thinking on how it will feel just to kiss you. But over all he will be happy just to hold you.

If all he want is sex the second time the two of you meet, he will start giving you hint, or maybe talk about little things when it comes to sex. Just use your better judgment and you will see a wonderful result.