Red-legged Frogs will be found primarily along the coast, and are more common north of the San Francisco Bay due to destruction of their former habitat.
The Foothill Yellow-legged frog occurs with Red-legged Frogs along the north coast, and the central coast. Red-legged frogs have disappeared from the Sierra Nevada foothills and most of Southern California, while the Yellow-legged Frog still occurs in some foothill streams in the Sierras, especially in the north. The Foothill Yellow-legged Frog is no longer found in Southern California, while the Mountain Yellow-legged Frog (which is probably a different species from the Sierra Frogs) hangs on in only a few small locations.
Yellow-legged frogs have yellow on the undersides of the legs, while Red-legged Frogs have red underneath the legs, but it is not always necessary to pick up a frog to identify it.
Red-legged frogs have prominent folds along the side of the body, and a dark mask through the eyes, while Yellow-legged frogs have indistinct folds and no dark mask. Adult Yellow-legged frogs are also smaller - up to 3.5 inches long, while adult Red-legged Frogs get up to about 5 inches in length.
Also, Foothill Yellow-legged Frogs tend to prefer to jump in water and sit on the bottom, relying on their cryptic pattern to hide them, while these other frogs tend to either hop away quickly, or dive into the water and swim away quickly.
There are two very similar types of Red-legged Frogs found in California. The two species meet just north of the Bay Area, with the Northern Red-legged Frog north of there and the California Red-legged Frog to the south.
If you think your frog is a Red-legged Frog, look at the range maps of the two species.
If you think your frog is a Yellow-legged frog, then look at the range maps of the two species below.
Geographic location will help you identify which species of Yellow-legged frog you have. The Mountain Yellow-legged frog occurs only in the Sierra Nevada Mountains (and a very small number remain in some of the Southern California mountains). The Foothill Yellow-legged Frog is found in the foothills of the Sierras at lower elevations than the Mountain Yellow-legged frog, and is no longer found in Southern California.