[g-clef]

As a lot of other commenters are pointing out: there's an adversarial and a non-adversarial way to do this.

When I'm interviewing someone I will often preface groups of questions with "have you worked with {x}?" or "are you comfortable answering questions about {x}?" before asking the actual questions covering that area. A simple intro question like that goes a long way towards keeping an interview friendly. The interviewee can say "no" to a small-sounding question without feeling like they're embarrassing themselves.

For example, they may be comfortable with Python but not super-comfortable with the GIL/multiprocessing/multithreading. If I ask "have you worked with Python's multiprocessing or multithreading before?" that comes across as much less adversarial than asking them to describe Python's GIL and forcing them to say "I don't know."

Doing an intro question also allows them to give a partway answer like "I've touched it before, but I wouldn't call myself an expert". If they say something like that I can re-focus my questions to another area, or just ask higher-level questions, rather than hitting them over the head with a deep question about something they've just told me they're not an expert in.

You do want people to be honest about their own limitations, but you don't want your interviewee to think that your organization is driven by macho posturing about being King Nerd.