How to tell a baby budgies' sex?

It’s a female, if any baby budgies (under 4 months) have any white-ish rings around their nostrils, then it’s a female.

I found this online:

In general, the color of a budgie’s cere is the simplest indicator of sex in adult budgies: males are blue, and females are white, light beige, reddish, or brown. While it is fairly accurate, the color of the cere cannot always be used to sex a budgie.

Other visual sexing methods include females having an almost invisible, whitish rings around their nostrils, and males having more rounded, bulbous ceres, both of which are often hard to recognize to the untrained eye.

And many people also say that females bite harder than males and that males have rounder heads while females heads are more flat on the top.

with a baby budgie, the cere will be pink in a male or baby blue in a female but editor is right about the adult colourings. The baby blue of a young female and the blue of an adult male are very distinctive as the baby female cere is very pale and an adult males cere is quite a bright blue.