Bullfrogs were my original favorite frog growing up.
Notice how the fold wraps around the ear of the bullfrog. If this were a "green frog", the fold would be a line going halfway down the back.
They can get quite large.
This picture is from the nearby park, my bullfrogs are very secretive and I don't have any pictures of them (yet!)
In the spring I bought 10 tadpoles thought to be "green frogs" and 5 tadpoles thought to be bullfrogs. The bullfrog tadpoles were bigger than the green frog tadpoles and took a little longer to emerge as frogs.
In smaller frogs, it's hard to tell the difference because they are skittish and tend to jump in the water before you can get a good look at them.
The bullfrogs are really secretive...I think I have two of them and the one at least is huge!