The black coloration is more dependent on the individual fish{where it was collected, it's host anemone, if it was captive bred,etc.}; the only way to really tell the difference is the number of dorsal spines. The percula clown normally has 10 dorsal spines and the spiny part of the dorsal fin is shorter.
Although PoukieBear was right in saying that percula's can have more black on their body.