Well what you would need to look for to tell a male from a female is that the females have what is called an *egg Spot* you need to know that male bettas DO NOT HAVE EGG SPOTS. Egg spots are basically a white dot that is found under the females belly.
This is a picture of an egg spot:
That is the ONLY way to be absolutely positive that it is a female.
Males fins are usually long and fluent. Though females CAN have the same thing (usually unlikely) Though Plakat males and Halfmoon PLakat males have short fins like the females. That is why you must search for the females egg spot.