she will have vertical stripes.
but why are you trying to breed? bettas aren't guppies. they're complex to breed, and you'll need a few more tanks than 1 20 gallon to raise any fry that don't get eaten by the parents. females must be removed away from the eggs straight after all the eggs are in the nest, and the male must be removed once the fry are free swimming. and then theres the feeding the babies, and housing them in seperate containers once the boys show aggression...and of course breeding pet store VTs is frowned upon anyway, you don't know how old the fish is, or what kind of fry you're going to get, if it's mutt VTs you'll have a hard time giving them away, especially if you get super lucky and get a spawn of 250-500.