About the only way to sex a terribilis is buy a darken throat patch. The males have a darker vocal sac. This ia due to calling, so can't tell until the get close to reproductive age, and have called for a while. Sean Stewart wrote an article in Reptiles several years ago about these guys, but don't know the issue right now. The only other way is if it lays eggs then its a female

I will try to find what issue it was.
Hope this helps,