well, the best you could hope for is that it has a nice firm belly (no sunken in areas), no obvious marks or injuries, a nice coloration, the ventral fins should be nice and long, not broken off. And of course swimming around, not laying on the bottom or at the surface and of course not lying on it's belly or doing a headstand! Whenever you go in, ask how lond they've had them (my lfs will not release any fish until they've had them for 3 days and they all check out healthy.. if even just one gets sickly or dies they hold them longer). Also, they SHOULD have some sort of warranty, even if it's just a few days.