look for the ones that are swimming happily, not chilling at the top or bottom. if they wiggle up to you, that's a plus. if you put two cups next to each other, they should flare(though i've had some that ignored their neighbor. xD).
stay away from any that look bloated, have pop eye, or are covered in ick(white specks) or a cottony stuff.
though, my newest male was chilling at the bottom, but he looked at me and wiggled when i picked him up. xD