One is Nemo and the other is Nemo's dad.
Just kidding.
I have read that you can tell the difference between the ocellaris and the percula's by counting the little spikes on there top fin. One will have 12 and the other 13 (I think, it was a long time ago I read that). Try googling it and see if you can find something. Bang Guy would know!! You could always send him a PM.
Would it change how you keep them if they are not what you expected?? Normal rule is to not mix the species of clowns but people do it all the time with those two. kinda hard to tell the difference if you ask me.