I'd look for a firm well-figured girl, nice clear eyes, no tears in her fins, active in the container, good coloring, not too "eggy".... then she should be healthy.
Personally, I wouldn't advise a chain store like petco and petsmart. I have had poor results with fish from both stores. They also tend to sell older fish. You'd be better off finding someone in a forum like this, who you can purchase from, or finding a reputable local fish store, where all they sell is fish.