If the Betta is still young before the body color develops, you can check the stomach against a back-light whether it has a "cone" portion extended from the stomach area toward the back.

Usually the case is that you can tell from male or female by the ovipositor, stripes, and the size of the mouth and the head of the betta. Females tend to have smaller sharper heads and males with larger wider broader mouths and a larger head. This is from my experiences. Sometime ago I had a female that looked like a male, but I knew she was female due to these clues