1. Male and Female parakeets are easy to tell apart. But you can't tell untill their Sexually mature. This means that they are old enough to copulate and produce offspring. Most birds become sexually mature at 6 months and older.
2. When sexually mature, Male (See, Benjamin, Blue one, below) birds will have a bluish to purplish cere. During mating season the male's cere will become a deeper blue.
3. When sexually mature, Female birds (See Kito, Above, Yellow one) will have a Pink to lightly purplish cere. During mating season the Female's cere will become Brown and flakey.
4. Immature birds will have a beige colored cere as you can see below:
The cere will slowley change to the color of whichever sex the bird is as it gets close to six months or older. A good trick to get a educated guess on what your bird's sex might be,
Male immature birds tend to have a pinker cere when they are younger
Female birds tend to have a more Beige colored cere.
This is not always right but sometimes works.
5. Sakari here (above) looks like a girl in this picture. This was when he was still immature, and Sakari turned out to be a boy! You can easily get misleaded if you try to determine the sex when your parakeet is still sexually immature. It's best to choose a name that fits both boys and girls, unless your prepared to have to change it's name!
6. Some male parakeets, like yellow ones or pied(keets with random bands of colors on their chest) can have purplish to pinkish ceres that can be misleading. If it's leaning toward purple it's probably a male. If it's pink or more beige or brown, it's a female.