I don't know how to sex parakeets myself. I keep Lovebirds and Cockatiels. But you can post this question with pics in the Caged Birds section of the forums. Lots of parakeet owners there.
Hi there!
I'm actually quite knowledgable with psitticines. How old are these birds? A mature male will have a blue/purpleish cere and a female will have a peach cere. If a female is in estrus the cere will be brown & crusty looking. Immature birds all have a peach cere. I can't tell the exact cere color from this picture. It looks like the yellow budgie might have a blue cere but it is hard to tell. Sometimes it is really hard to tell on these color mutations. I hope this helps.