the false perc your referring to is an Amphiprion ocellaris.
Dorsal fin spines: The True Percula Clown (Amphiprion percula) typically has 10 dorsal spines. These are the spines in the fin on the very top of the fish. The False Percula (Amphiprion ocellaris) has 11 dorsal fin spines. On rare occasions the A. percula will have 9 dorsal spines and the A. ocellaris will have 10. However, in general, the True Percula will have 10 and the False Percula 11. Also, the back part of the dorsal fin is generally taller on the False Percula (Ocellaris) than it is on the True Percula.
Black outlines: False Perculas often have much less black outlining their white strips. Sometimes it may look as though there is no black at all, but there is almost always at least a thin black outline adjacent to the white stripes. The True Percula, on the other hand, usually has thicker black stripes outlining the white stripes on their body. This is generally true of standard clownfish, although colorations and patterns can vary greatly from one fish to another. Therefore, this is not a true test one way or the other. It is a good piece of evidence though, when taking into account the other differences.
Eye color: Another way to tell the difference between the two species is often the color of the eye. On the True Percula, the eye shows more orange color to it surrounding the pupil. Conversely, the Ocellaris clownfish has more black surrounding the pupil.