It is impossible to do so with baby parakeets. However, once they reach maturity, normally six months old to a year, it is simple. A male's cere will be blue.
This is very difficult to do. You can attempt to spread the down feathers and examine the animal but the gender differention in baby Parakeets is minor. I suggest waiting until the animal is older.
1)observe the behavior of parakeet. Females are aggressive and they try to nest. Males will sing and whistle. Males are more active than females. 2)By checking the DNA sex can be determined.