i have always known it to be the color of the band right above the beak,,: if it is blue color it is a male and if it is a pinkish color it is a girl The band can also be a purple color in which case...
The male (Boy) parakeets have a BLUE nose/cere (right above the beak)
and females have a PINK or BROWN nose/cere, and babies are white, purple or in between so you will not be a ble to tell until...