The best way to determine the sex of any bird is to have a veterinary expert evaluate the cloacal opening of a bird. By inserting a lubricated finger into the bird's vent, the vet can attempt to palpate a penis, which, if present, indicates that the bird is a male. This is an unusual technique for determining bird gender, as only about 3 percent of the living bird species possess penises. Alternatively, the vet can cause the penis to evert by applying pressure to the proper portion of the vent. This is a potentially dangerous technique, which should only be performed by a veterinarian.

My experience has been that Wildlife Biologists and Techs can also do this. Never tried it myself.
As an older man, I know what it's like to have a finger up my vent.