well, before i go in the store i have a general idea of what i'd like to find, and reasonable parameters for how much i'd be willing to vary on that desire. for instance, if i'm looking for a mustard gas HMPK, i'll also be checking out any HMPK's that are either blue OR yellow, rather than the MG combination, as well as looking for MG's in other tail types.
after i've found one that's who i'm looking for in terms of finnage and color, i try to gauge their general health. i look to see if he's active, swimming straight-and-true, any visible fin or scale damage, fungus etc.
if he checks out as a healthy fish, i get an idea of his personality. will he follow my finger around the cup? does he swim towards me or away from me when i look at him? does he generally seem like the kind of fish who'd be something of a companion, rather than just a little scaly dude who lives in a watery box in my apartment?
if he's good on all three, i beg my fiance to let me set up another tank. once she accedes to me, i have a new fishy friend.