Males usually have longer fins (unless they are plakats) and have much more vibrant colours then the females. The males also blow bubble nests (which is used to store the eggs, and the male looks after the eggs) and the female can become gravid (have eggs inside her) however females will reabsorb their eggs if there isint a male to mate with. there is some other differences that other members can tell you about, as im not to sure on the others, but generally lfs will stock male betta's.
Female
Male