Well this is pretty simple. The clue to the difference in gender is their tail. Us their tail.
Female betas always tend to have really short tails. If the container that you get the fish in at the pet shop says "male" on it, and it has a really short tail, then you should know what gender it is. Don't let the container fool ya!
Male betas are the opposite of the females. They have long tails that drag behind them. Their tails are about the length of their bodies. And again, don't let the containers fool ya. The pet shop keepers tend to mix them up.
So now you know how to tell a male beta from a female beta. Enjoy your fish!