How can you tell if a male or a female betta fish is an adult betta or a baby betta?

No female bettas have the long flowy finns commonly associated with the betta fish. Some males that are a specialty type (Plakat(sp?) types) have short short finns exactly like a female but they are fairly uncommon and well labelled usually due to their rareness.

Usually when you get them from your local pet store, it says on the cup you got it from if its a boy or a girl. Otherwise, I am not sure.