Hi there
The only 100% surefire way to determine the sex of a parakeet is to have a DNA test done. There are some ways to tell but they are not always 100% accurate but pretty close.
The cere (part above the beak) will turn blue for males and a brownish red for females. If they are babies (younger than 4 months old the method for testing is slightly different.
I am going to attach a photo so that you can see the difference of the ceres. Remember that the only completely accurate way to determine sex is a DNA test.
http://www.kiva.net/~daylight/keet.fix.jpg
You can also check out this web site that will instruct you on the difference in very young birds.
http://www.kiva.net/~daylight/budgie.html
I hope this has helped you to find an answer to your question. If you have any further questions please don't hesitate to ask.
Sincerely,
Bunny -- Veterinary Technician.
If you feel I have answered your question please hit the accept button so that I may be compensated. If you have any further questions or need further clarification please let me know. I am not happy until you are happy. Thank you.
Veterinary Assistant
10+ years at a veterinary clinic as a veterinary assistant. 20+ years raising/showing rabbits