When looking for a good betta at the petstore, remember to look at the eyes. They should not be cloudy or enlarge. Try to find a betta with perfect scales and no signs of illness. When looking at the fins, they full and not borken. Dont be alarmed if they are not active as sometimes bettas like to rest and often get bored of seeing the same fishes over and over again.
When I am shopping for a good betta i try to look at the coloration, if it is hard to find or just so common but to a higher degree, also if they have any fin characteristics that i like. When shopping for VTs i like long and thin dorsal fins with extentions on the back side. The anal fin should be deep and have rays extending by the tail.