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.
Female P. terriblis are one of the few female darts that can call too. I would have to say you need to observe behavior more than anything. Although not always true I usually find males develop smaller and slower than females of the same age and feeding habits.
Thanks for the replies. We are going to assume female unless proven otherwise. I guess we'll just get a few more and put them in a tank and see what develops! I love this one. She is so animated!! Much larger personality than my D. auratus or D. leucs!!