dont buy from a place that sells them in cups.
If any of them is dead, dont buy from them.
Reputation is a good rule of thumb.
Betta should be active. If theyre lying down, wait or use a mirror to have them flare. If their fins are clamped, dont buy.
If buying online, definitely reputation first, then whoever has videos as some breeders upload videos of the actual fish, then photos of bettas form. You should look for or confirm you get the fish pictured, not an equivalent.