There are a number of different ways to tell, but the most obvious is that the fins and tail are huge. Females have little fins. Google image male betta, then female betta, to see the primary difference. :)
To help him heal up, make sure the water stays amazingly clean - whatever your water change schedule is now, double it - and keep it warm, at least 80F.
If you are feeding only freeze-dried bloodworms, you need to switch him onto a pellet diet (try New Life Spectrum - widely considered to be the best). Bloodworms are high in fat and should only be treat food.