Usually you can tell by color but the only true way to tell, no pun..., is the top fins of each species. True perc i think has 11 spines, while a false has 10. Not sure on those numbers. True percs are more pricey as well.
Actually sometimes they can both have same colours and can both have 10 dorsal spines. I m not really sure if thats true but i heard the best way to tell the difference is the circle around the eyes' of true percula are orange and ocellaris' are brownish-darker. You can see this on Melody's pictures also
Actually i read this both can have 10 spines thing at http://www.globaldialog.com/~jrice/f...ge/percula.htm
but it says "in rare individuals", dont know which website to thrust these days
A. percula has 10 (but in rare individuals 9) . A. ocellaris has 11 (but in rare individuals 10) fin rays