You could ask the people to feed them, and pick out the one who ever seems most excited for the food. Or if you put your finger to the tank, and some of them come forward rather than hiding...maybe.
Also judge by the coloring, if some appear more pale, don't get it. If some are dark(not solid black, but just bright blue), those are probably happier(more dominant), and healthier.
When you get him home to his tank, make sure he is the most dominant fish in the tank(if he isn't he'll stress). And make sure he is eating.
I have one who is pretty picky, he really likes frozen food but doesn't care much for the pellets, and prefers flakes over pellets...
And I don't know if this is relevant to everywhere. But I've noticed the ones who don't look as healthy usually are more expensive than the healthier looking cheap ones. So maybe just look around a lot, and try to pick out some happy healthy looking ones.
I was lucky, I bought mine on a whim. But he ended up being from the healthiest(and cheapest), batch in my area.