You might be able to do this via X-ray, but from what I understand, it would be tough without the presence of eggs as X-rays aren't great at showing soft tissues (reproductive tissues in this case). I could be mistaken, but something with greater resolution (like an MRI) would probably be better.
IMO it is very challenging to determine reproductive organs (and most other soft tissue) on X-ray of a tortoise. I am sure there's a radiologist somewhere who may be able to do better than a guess, but I don't know that there is much available for comparison as far as gender specific radiographs on young tortoises go. Just curious....what's the rush? I kind of like the "late in life" surprise element of tortoise gender : )
Blood typing or endoscopy, possibly and MRI or ultrasound. Here's an example of the problem with xrays: http://bit.ly/A5Ru so unless they're gravid there's nothing for the xray to pick up gender wise. Still, these are fun to look at: http://bit.ly/yxxGiY
I have been thinking about getting another leopard. I do not want to have babies. I want to house them together, of course unless they don't get along, I can separate. However I don't want to separate because of one being male and one female. I wouldn't think one XRAY would be harmful. I guess it doesn't matter now, if it won't work anyway. How stressful would an endoscopic test be on them? Also would an MRI work and be better, less stress?