Generally, the best way to identify the sex of a bird for breeding is through an endoscopic evaluation, conducted by an experienced endoscopist. A doctor uses an endoscope to enter a natural orifice to determine the sex of a bird.
Laparoscopy (in which an endoscope is inserted through an incision), along with chromosome analysis of blood feathers, provides data about the condition of the reproductive tract. It can detect some chromosomal defects that could affect reproduction.
For more information, read Sex Determination of Psittaciformes