How do you know if your boyfriend is using you for sex?

Hold back. See if the guy has any real substance. Talk to him, see if there is anything in his head that's not affected by hormones. The problem with jumping into bed too quickly is that you never get to figure out what is in the other person's head. Is he a budding physicist? Maybe he's a brain surgeon in the making. Maybe he's just a stupid gang-banger and won't ever amount to anything.