The actual literal differnce is that the ocellaris (false perc) has 11 spines in his dorsal while the true perc only has 10. But counting those is all but impossible.
Commonly, the ocellaris looks like "nemo" while a perc (or true perc) looks like a clown with to much black..
From my own experience, true perc are meaner than ocellaris. Course it seems that the darker they are, the meaner they get, with all clowns.