Pictures will help. According to the field guide to South African frogs, it is fairly easy to tell the difference between the giant and the dwarf. However, recently there was a fourth species of African bullfrog that was rediscovered which has been causing a lot of confusion in the pet trade.

The giant:
The distance from the eye to the tympanum is about twice the diameter of the eye (key feature)
Armpits and groin yellow

The dwarf:
Upper jaw with irregular vertical pale bars (key feature)
White, elliptical or crescent-shaped spot on tympanum (key feature)
Pale interorbital bar (key feature); this is a bar located on the top of the head that connects both eyes
The distance from the eye to the tympanum is about the same as the diameter of the eye (key feature)

It is fairly difficult to tell the difference if the frog is a juvenile.