no surefire way, besides venting and checking the breeding tube.
breeding tube will be blunt and fat if female, skinny and pointing forward if male. vent will be shaped like a U if female, V if male.
besides that, for ways that arent 100%, but have worked for me, there is: fatter belly on females, males tend to be lighter, males grow faster, and Supposedly, the males have less of the parrot notch on their head because they allegedly will have a small hump(although i don't believe this one).