So I have this "female" that I bought when it was a few weeks old. Anyway a few days ago I saw the while thing under the belly which then I was like it's a female. Today though...it blew a bubble nest and is acting like a male.
It does look like a female due to the body shape and females do blow little bubble nest sometimes. However, the longer pointed fins suggest a male betta. The only sure way to tell is to look at the egg laying tube. It should be near the front of the analfin.