Xawp, I believe you are correct. True percs have 10 dorsal fins and a false perc has 11. True percs are more orange then false too. There is something about pectoral rays too, but I don't think you can judge the difference between them by that.
I've also heard and observed in most cases that true percs have larger black margins between the orange and white sections. False percs tend to have very thin black margins or none at all.
I've always found it hard to do the whole dorsal ray counting thing... the suckers won't sit still long enough for me to get close enough to count or get a decent picture to count... so I usually give up and go with coloration.
Amphiprion percula -
Has 10 (rarely 9) dorsal spines.
Usually has jet black margins of varied widths around its white bars, often of which can be rather thick.
Distribution of this species in nature: Northern Queensland and Melanesia (New Guinea, New Britain, New Ireland, Solomon Islands, and Vanuatu).
Amphiprion ocellaris -
Typically orange in color with three white bars, with the middle bar having a forward-projecting bulge.
Has 11 (rarely 10) dorsal spines.
The spinous (anterior) part of the dorsal fin is taller.
May have no black margins present, but most often has thin, never thick black margins around its white bars.
Distribution of this species in nature: Andaman and Nicobar Islands (Andaman Sea), Indo-Malayan Archipelago, Philippines, northwestern Australia; coast of Southeast Asia northwards to the Ryukyu Islands.
For what it's worth, I've seen true A. percula with nine dorsal spines and no black as well as ocellaris clowns with eleven spines and as much black as usually associated with the perculas. I wouldn't say that colouration means anything between these species, at least not when dealing with captive-bred examples.
That said, I would ask why this particular species differentiation matters to the average hobbiest...while I'm certainly an advocate of discovering exactly what one has in their aquarium, the vast majority of the perc's/ocellaris purchased these days are captive-bred.
I would consider the vast majority of captive-bred perc's and ocellaris clowns to be hybrids rather than distinct species these days. I can order a batch of Ocellaris or "True Percula" from ORA / C-Quest / ProAquatix - they always come in as a mixed lot. Some fish in the bag have nine spines, some have ten, some have eleven. Some are orange, some have more of a yellow tint, some are more red...some are misbarred, some have more black than others. While wild-caught Perc's are generally much more aggressive than ocellaris, there doesn't seem to be any difference in the tank-raised ones.
In my experience, there's no real difference between captive-bred Ocellaris and Perculas.