Nowadays, it is very difficult to tell, especially with a fish that has a dark iris around the pupil. In addition, there are many inter species breeding between occ and perc, hence the name percularis. I have some Occ/Perc pairs at home, and they will spawn. Just make sure if you plan to sell or give to someone they know they are getting a hybrid. Some people out there have hybrids and don't even know it. I have seen occ and perc eyes out of sibling offspring. These days a lot of the newer designers have a hybrid parent somewhere in the blood line.