If he is a Jenday than I believe the only way to find out his sex would be DNA or surgical sexing. I always go with DNA sexing, though.

Jendays aren't hybrids. I have heard of Sun & Jenday hybrids, though. Sunday Conures? Either way, I am not too familiar with them. I would assume it would be the same case, though.

Some people say you can tell males from females by their pelvic bone. Maybe I am incompetent, but it never works for me. :] Plus, it helps if you have another bird of the same species to compare with.

Good luck!