You ask them about a previous project they've worked on, or a problem you currently have and take it from there. You are bound to hit interesting problems where you can ask them what they would do, if they've had similar issues, how they solved it, what tech they used and why, would you do it differently now, etc.

If you care about performance and data structures you can dig in, you can ask what bottlenecks they had, what structures they used. I've had a lot of success just asking in the middle something like "why not use a list or vector?" (in C++), the discussion that follows tells me already if this person knows how memory works, how algorithms scale, etc and this are things I really care about in my industry, not just random trivia.

All of this is done as a conversation, I want to minimize their nerves as that just obfuscates the real person I'd be working with.

I get none of that info asking someone to invert a binary tree on a whiteboard.