It is very easy to know if you are looking at a tree frog or a bullfrog.
Tree frogs are very small – 2-4cm long. They come in two colour phases – either brown with dark blotches or bright green. They always have a black stripe running from their nose past the eye and down to the shoulder. They make a “ricket-ricket” sound way out of proportion to their size http://www.naturesongs.com/frog3.wav
Bullfrogs make a quite different sound, rather like a rapidly repeating foghorn http://www.learnnc.org/lp/media/collections/fieldguide/frogcalls/bullfrog.mp3 They are far bigger than tree frogs – up to 25cm and can weigh up to 2kg.. They expand like balloons when attacked and have teeth on their lower jaw to hold their prey. They eat almost anything. They were introduced from Africa and are a thoroughly nasty species in this area.