Mature males have a white lump behind the front arm on the side of the body
Got this info from
http://www.pipidae.net/david/Page2.htm#malesfemales
Males and Females
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.
More info:
http://www.fishpondinfo.com/frog4.htm#dwarf
Lots more info :
http://www.google.com/search?sourceid=navclient&q=african+dwarf+frogs+++sexing
