You don't want to lose the upper hand in the dating dynamic, but you want to move things forward when it's right. When it comes to giving up the goods, one false move could really compromise the future of the relationship. There's no fail-proof formula, but here are some pointers to keep in mind when judging.
1. If you're in it for the long haul, hold off
If you think he wants to make you his girlfriend, and if you want to make him your boyfriend, it's generally best to hold out for as long as you can. The first few dates are like interviews - pass them like a lady and show him you're girlfriend material. This will also give you a chance to decide if he's boyfriend material. After you have sex, the "courtship" and innocent flirtation might not be there.
2. Consult some spies
If you met this guy through friends or have mutual friends who are totally on your side and you can trust no matter what, ask them to find out on the sly what he's said about you. This will give you better insight as to where you stand. You may also want to find out a bit more about his dating track record, just for your own personal files.
3. Check your chemistry
Are you two revelling in saucy banter, laughing at each others' jokes and struggling to keep your hands off each other? If yes, good. That's what you need to have awesome sex. Of course, that all takes trust, familiarity, and confidence. In other words, it takes time to get to that place. Make sure this isn't something you "let him do" on the first couple of dates, but something that you do together.
4. If you wouldn't feel right talking about it, don't do it
Here's a handy self test: ask yourself if you would you feel comfortable talking about sex with this guy. If no, you're probably not ready to do it with him just yet. If you reckon you'd both be comofortable talking about sex and maybe even past relationships together, chances are you've built enough trust to get down and dirty.
5. Go with your gut
At the end of the day, when it feels right, it just feels right. Don't let that inner voice be drowned in booze or seduced by your partners sexy charms right before you make the decision, either. If you're really doubting yourself / driving yourself crazy questioning it, it's probably too soon.