I would not seperate the frogs like the previous post said they are social... also be weiry of putting small fish in with them because they will not hesitate to eat what ever is in their way.... the only way i know of to tell gender is the size of the legs... females have much bigger legs... here is a more indepth post "In H. boettgeri the males are slim and when mature they will develop a small gland behind each of their front legs. This gland is called a post-axillary subdermal gland. It looks like a small pimple. These glands apparently play some part in mating, but their role is not well understood.
The females of the species tend to be a bit larger (about 20%) than the males. They will become
almost pear-shaped when mature and their abdomen fills with eggs. The females typically have a slightly longer tail than the males.
The males of the genus are the only ones who "sing" or "hum." Like most frogs, the singing is designed to attract a mate. The males will sing even if no females are in the tank. If your dwarf frog sings, it's a male. In H. boettgeri, sexual maturity begins at about nine months of age" and here is a great site that will answer alot of your questions Dwarf Frog FAQ