ok, didn't know there were ocellaris in the tank. I'm sure you can understand the confusion. The easiest (and least absolute) way to tell is that percula tend to have more black between the white and orange parts than ocellaris have. The other way is by looking at their eyes, ocellaris eyes tend to look a bit muddier and have brownish areas around the pure black pupils where percula tend to have very orange area around the pupil which make the circle of the pupil look much more distinct. The best way to tell the difference is to count the dorsal spines but that is not likely to happen in the tank with a bunch of other fish.