"You find yourself on a box. Maybe it's Linux, maybe it's Unix; it's definitely Unix-like. How do you figure out what operating system you're running?"
Now here's what I love about this question: there's no longer one correct answer. There are literally hundreds of them.
It gives me a chance to be super encouraging; it impresses upon the candidate that this isn't a me vs. them moment.
So far the most creative I've seen was "I'd use nmap to fingerprint the box." That one was awesome!
"I'd check /etc/issue." What if that file doesn't exist?
"I'd turn it off and see who shrieks about the X box. X will tell me what OS it is!" Points for creativity!