Just ask anyone in academia? It barely counts as an open secret. Most admission forms (for the Cali and other state schools/private schools) would ask you explicitly if you have any relatives working for the school and their specific title and role.

Definitely not true for UC. Had a friend whose mom is a professor there. There wasn't anywhere to indicate this.

Privates ask about alumni connection, and possibly about teaching connection. But not the public schools.

Regardless what you've mentioned isn't proof that this is why so many get in. The fact is that kids of PhDs are likely to have genetic and educational advantages. That is likely much of the reason that they get in at a higher rate.

It is rampant in private schools. UC is a public school.

I mentioned UC because GP claimed public schools in CA do this. They don't.

I checked Stanford's admission forms [1] and there is nowhere to even report this. They link to the Common App and don't have forms that ask about where your parents work.

1: https://admission.stanford.edu/apply/first-year/index.html

Check Stanford and USC. It is a well known (though unofficial) policy.