I know how you feel...when we adopted our 2nd dragon our friend said it was a boy (hence the name Daedalus). I checked the tail (a 2 year old dragon), couldn't see any male bumps, kept asking, are you sure? I think it's a girl...yep I'm sure! I was still suspicious that Daeds was a female, and sure enough a few months later she laid eggs :roll:
edit - as for your pic I don't see any obvious bumps, but try setting him/her down and bending the tail up and back a bit, that should give a better angle if there are any bumps.