I'd say avoid fish which look like they have frayed or tattered fins. If they look queezy, week, or pale, also avoid buying them. Look for things like fungas, fin rots, ich(white spots), parasites such as ticks, etc. Check for any abnormalities such as bulges or lumps in wierd places.
I remember hearing something about checking their bellies. I can't remember what it was. Something to do with it being a healthy white and not pink or something.
PetsMart will only get their fish from sources they know they can trust, so they claim. They do not accept babies from local hobbiests.
, never buy any fish which have been dyed. These fish are usually marketed as names with the words "painted" prefixed to them or something fruitty like "fruitloop," "strawberry," or "blueberry" etc. They may look pretty, but it's not pretty how they get them to look that way.