I imagine your vet would need to 'probe' to find out for sure. I have a snake that I believe to be male as he has a long tapering tail as opposed to the stubbier ones that females have, but I have never had him probed as its a stressful procedure and as I don't intend to breed him I don't need to know!
The only way my friend found out that her iguana 'Eddie' was female however was due to her becoming ill a few months after she got her, she was egg-bound! Strange thing is she has all of the exterior characteristics of a male iguana! If I'm correct bearded dragons have the same appearances for males and females.