I feel stupid for asking this but I was given a baby Betta and I think it's girl but I can't tell. Here is a picture of it.
http://1.bp.blogspot.com/-lIt6r4Y012U/Tb-i3O2TVjI/AAAAAAAAAHU/9TXa-TPzGAw/s400/Male%2526Female%2BBetta.jpg
Looks female to me. Check out this comparison image.