You could determine the gender by examining the fire-bellied toads, at the pet store, for nuptial pads. Only breeding males develop nuptial pads, or rough dark areas, on the inside of their thumb, second finger, forearms and feet when they are breeding.
Compare the sizes of the FBT too. If you notice that certain toads are plumper and rounder than others, these may be the female toads. Males tend to have a more slender appearance. Also, compare the forearms of your toads. Males tend to have more muscular forearms than females.