Adult parakeets are sexually dimorphic, the differences between the looks of the males and females, only by the color of the cere -- that body part above their beak where their nostrils sit. Boys are blue (how convenient!), girls are not-- their color can vary but is often tanish or brown. The cere, an ambiguous pale color at birth, will settle into its final color when the keet reaches maturity, somewhere around 8-12 months.
When we increased our keet-load from two to four late in 2009, we got two more girls (assured by the pet store), so we would avoid the confusion that we felt sure would erupt in a co-ed cage. Now, as they are approaching maturity, we find that the cere of Newfie, our blue keet, has turned from pale to bright blue. Looks like she might be Our Little Man!
We have read that the cere can turn blue and then closer to 1 year turn another color (or vice versa), so it is not certain that we have a transexual keet (transexual at least from our point of view, as "she" probably knew the status of "her" genitals all along). We'll have to wait and see. But you can be sure we are watching her "sister" and store-mate, Baja, very carefully!