It depends on your birds mutations (which is often not straightforward with budgies) as to what colour the cere will be. This is a great website and should give you some information to help you figure it out:
https://www.cutelittlebirdiesaviary.com/budgie-genders--3-weeks-to-adult.html
https://www.cutelittlebirdiesaviary.com/budgie-genders--3-weeks-to-adult.html