You could ask them how to test an elevator? 
Just kidding! Ask them whatever seems fit for your organization and something based on his experience.
Let’s say he knows Selenium, you could ask whats the difference between asserts & verify. But the real questions is: How does Selenium work? You want him to know what he’s using and not just someone who uses any framework without knowing what it does.
It really totally depends on the person who you get in front of you.
Our first interview is mostly a personal interview, to see if he/she fits into the organization. 2nd interview is a technical interview. We’ll challenge him/her into the beyond!
It does not matter if he cannot answer all the questions, it matters if he’s willing to learn and being able to admit that he doesn’t know everything and should look it up. (we do consultancy, for context)
Ask them to draw his favorite or previous test automation architecture landscape and see what he includes. Crappy image below but some pointers to think about. If he only draws his test frameworks and nothing about logging, reporting and test data, …
