There's really only one good way to tell the difference, and it always seems to work (I'm a guy and it works on me). If he really loves you, he'll wait before he sleeps with you if you tell him no. I don't mean you should just be like "NO". You can still show that you have in interest in being intimate, but that you'd be comfortable getting to know him first, being friends, etc. Otherwise, if you just go and sleep with someone, you won't know until later if it was love and lust or just plain ol' lust. Usually later means after it's too late not to get emotionally hurt.