It is very difficult to tell the difference between giant and dwarf juveniles. They are nearly identical. I used to have a giant bullfrog, but now have two dwarf bullfrogs (Pyxicephalus edulis). There are a couple of key characteristics that may help you to determine which species they are. The dwarf juvenile and adult will have a white spot in the tympanum (ear drum), while the giant doesn't. The other characteristic is the distance between the eye and the tympanum. In the giant, the distance between the eye and tympanum is about twice the diameter of the eye; the dwarf the distance is about the same as the diameter of the eye. This information comes from: A Complete Guide to the Frogs of Southern Africa by L. du Preez and V. Carruthers.