The guiding principal behind any question to a job applicant is: “Can the employer demonstrate a legitimate job- related or business necessity for asking the question?” Both the intent behind the question and how the information is to be used by the employer are important for determining whether a question is an appropriate pre-employment inquiry.
See a list of questions at Interview Questions – Appropriate or Inappropriate?
One of my least favorite experiences in my own days of being interviewed for jobs was when the interviewer would preface the questions with, “I know I am not supposed to ask this, but…”
It is hard to get past this one without being rude and showing your displeasure. I never could.
Today, I would just change the subject, but I am more fearless today now that I am my own boss.