I'm pretty positive that's a female, even with the blurry picture. It appears to have very short ventral fins, and generally even shortfinned males have longer ones. Your also has a short and rounded anal fin, which usually indicates a female.
To tell for sure you can try and find the ovipositor like midnight mentioned.
Size and shape can be a factor too, but I once had a female that was much larger than both my males and not fat in the front like my other females, yet it was most definately a female by all other factors, so I don't think that's a good indicator.
This site has a good demonstration on sexing bettas:
http://www.freewebs.com/ontariobetta/bettainfo.htm
To tell for sure you can try and find the ovipositor like midnight mentioned.
Size and shape can be a factor too, but I once had a female that was much larger than both my males and not fat in the front like my other females, yet it was most definately a female by all other factors, so I don't think that's a good indicator.
This site has a good demonstration on sexing bettas:
http://www.freewebs.com/ontariobetta/bettainfo.htm