Female can blow bubble nests and flare, just like males. Some females seem to not have an "eggspot" (ovipositor) and some males seem like they do have them. Fin length, body shape- those can vary between bettas and aren't definite ways to tell either.
What you want to do is look for ovaries. Female bettas have them, and males do not. Take a flashlight and shine it from behind your fish- if you see a large shadowy triangle behind their stomach extending towards their tail, those are ovaries and your fish is female! No triangle, just the round stomach? Then they're male.
I'm attaching a couple of pictures of females with their ovaries highlighted, and males with no ovaries. Try the flashlight trick with your betta and compare! (It's easier to see in person, I promise)