I usually start with asking them to describe the technologies in their ideal stack. That gives a good platform to talk about frameworks, technologies, and challenge them with the bits and bogs.

I'm looking for:

* Technical skills (on a rather broad level): how much experience do they seem to have? Are they interested in their field beyond work?

* How clear is their communication? Can they argue well?

* How well their preferences align with ours in terms of technology.