You can typically distinguish percula from ocellaris by the color of their eyes ... true percs will have lighter irises while the ocellaris have darker ones. You can also distinguish the two species by dorsal fin ray count, but that is a little tough to do from a practical perspective. As a general rule, if you see an orange and white clown with anything more than solid black banding around the white stripes, you can be pretty sure that it is a percula (except for Darwin ocellaris, which are predominantly black and white, sometimes with a bit of orange on the face). If the black banding is very thin, then you should look at the eyes since some percula will look absolutely similar to ocellaris (in terms of body coloration and pattern.