Please don't advise people to cycle their tanks with live fish. This causes the fish unnecessary stress and discomfort and can kill them. This is essentially placing the fish into a toxic environment and should be avoided if possible.
Definitely look for signs of disease on the fish you purchase. Increased slime coat, small white spots, white stringy feces, and heavy breathing are all bad signs.
A common birth defect in Captive Bred Ocellaris is bent or deformed gill plates. My first pair of fish (also Ocellaris, both appeared healthy at the store) turned out to be sick, and the little male has deformed gill plates and seems to be having a bit of a tougher time than the female. I have nothing to link his difficulties to these deformities, it could be a million other factors, but just an observation I've made.
Also a great idea to see them eat at the LFS, often times even if your fish end up being sick, if they're eating you'll have a much easier time getting them healthy again.
And, one of the more important things...don't let the LFS talk you into ANYTHING. If you're not comfortable with a purchase, don't make it. Wait to find the fish you really want.
Also, quarantine, quarantine, quarantine. I didn't QT my fish and it ended up being three times as much work to set up an emergency QT tank and transfer them once they started showing symptoms. It's crucial to QT all new fish for at least 6 weeks to observe them and if necessary to treat them.
Hope this helps!