If you find a Leopard Frog in California, the best way to identify it is by using the range maps, although it is always possible that an animal was released in an area where it does not naturally occur and won't be shown on the maps.
If you find a Leopard Frog in the extreme southern part of the state, it could be a Lowland Leopard Frog or a Rio Grande Leopard Frog, but the Lowland Leopard Frog has been, for the most part, extirpated from that area.
Here is a
Chart of the Features of Ranid Frogs found in California that might help you distinguish the several species of Leopard frogs found in the state.