The amount of black isn't a good test usually because it also depends on the age of the fish and individual variation. Regular orange A. perculas *usually* have more black and thinker black than A. ocellaris, but it isn't definitive.
Without killing the fish and examining the skeleton, the best visual method is looking at the ring around the eye. In A. ocellaris, it's muddy and typically greyish-orange. In A. percula, it's clear and usually bright orange.