Mostly, make sure she looks healthy and at least semi-active. Check out her fins to make sure they aren't really torn up or anything, and that they are open and not closed up. Closed fins=stress or sickness.
If she is in a tank, make sure the rest of the fish show no signs of disease. Also, watch when they try to catch her...she should swim away from the net and not just sit there and let them catch her easily. If she is in a cup, she should move around when you move the cup.
Color is an individual preference... Size: a bigger fish is an older fish...also an individual preference.