Like Siobhan said the DNA is the only method that is ~98% for sure and surgical testing is 100% accurate... everything else is basically just a theory.

There is the head slope theory which basically states that females have a very sloped head whereas males don't...
Click~Here for the pictures and more info.
Some vets say they can determine the sex by a pelvic examine since the females pelvic bone is farther apart. Basically people argue that that isn't really that accurate... it's all relative to what you're comparing it to (if that makes sense?! lol) Don't try this at home as you could injure the bird!

And then there is the differences in mating that Siobhan explained... but like everything else, that's just a theory too.

Hope that helps!