its difficult with juvies; the tell tale sign is the number of spines on the dorsal fin. but their juvies .. so the spines are small and sometimes not completely developed, and their moving around, making it even more difficult ...
typically trues have more black, but the black usually doesn't develope in until they're older
i think your only logical indicator is to use the shade of orange. its a very rough guess, but better than reaching in the tank blind. trues usually are a more vibrant orange.
if you want to be more accurate, then you'd be better served ordering from internet, or maybe phishy can set you up and special order you one?