Ask them to think up a way to implement a function to do X. Keep X simple, simplify further until you find common ground. Any other ways to implement? Tradeoffs? What about under this or that constraint?
Ask about some peculiarities of their favorite languages.
Have them describe a recent difficult bug (very insightful - in their description's wording, the scope of the bug itself, steps taken to solve, etc. - but could be 'proprietary').
If you talk about past projects, they talk about architecture and project management. If you ask about code they have written, they say it's all proprietary.