What methods did the vet use to sex the snake? Because ball pythons aren't sexually dimorphic. The males and the females LOOK exactly the same on the outside.
If you REALLY want to know, I'd see if I could do an ultrasound or another kind of imaging diagnostic. A lot of those will allow you to see internal structures. Some boa breeders have them to track litter development in females. I cant' imagine it would be difficult to find hemipenes on a male, but I've never tried it.
have you tried having someone pop the snake?