Female bettas have different tail types just like males. Its pretty easy to tell a PK male from a VT female.
Males will have longer ventral fins and should not have an ovipositor (egg spot). It is possible for some males to have this but it is NOT possible for a female to not have it so if you don't see an egg spot that is a male.
Look at pictures of male and female bettas. Their body structure is different, after a while it becomes pretty easy to tell one from the other... except for a few special cases.
*Be smart.. Do your research!!*