All bettas are different and depending on conditions of the tank, or possible disease it could be hard to tell. If you are putting him in a community tank it may be a good idea to see how he interacts if he is going to be by himself then it really doesn't matter. I would just try to find one whose fins are intact, and looks healthy. Also a fish in a cup may do well in a community setting I personally would just find one you like weather he is in a tank or a cup.