Found you a quick link with pics:

http://www.reptilecare.com/leopardgender.htm

After ~ 6 months, you should be able to sex them. Males have hemipenal bulges and femural pores. Females don't. Sometimes, they develop later than expected. The general rule of thumb is that, if you can ID a young, adult male, he's male. If you ID a young adult female, she's PROBABLY a female. (She might just be a late blooming male.)

You're right that males will be aggressive toward each other and should not be housed together. Females can be housed together. If you wind up with one male and one female (as unfortunately was the case with me), the general consensus is to separate them, to avoid aggression and over breeding. I've taken to feeding them separately, outside of the enclosure, because I saw some definite food-aggression, but I've been lucky enough that they don't (yet?) have any interest in mating, so until I see them trying to do that little dance, I'll keep them together. (They're about 2 1/2 right now). My male only has ANY interest in the female when she's competition for a cricket. Otherwise, they pretty much ignore each other completely.

There is also something called a "hot female," which is a female leopard gecko that results from being bred at too-high temps. These are quite aggressive, and should be housed alone. I have not yet been able to find a pic of their underside-bits, so I do not know if there is any visible way to sex for a hot female. (If anyone out there has any info on hot females -- besides the temps that create them -- I'm actually very interested in learning more, so please PM me.)

Hope that helps. I have leopard geckos, and I read way too much for my own good, so let me know if there's anything else I can help with. I think most of the basics are covered in the care sheets, though.