Ask HN: How do you interview someone more senior than you?

I work with a small team of developers, and my boss wants everyone to be involved in the interview process for new hires. This was great when we were looking for an intern, but now we're interviewing for a senior position, and I'm feeling a little out of my depth (I'm 100% self taught, with a degree in Political Theory. The impostor syndrome is strong).

How do you interview someone who will more than likely end up leading you later on down the line? What should I be looking for? My gut instinct is to ask process questions, but any insight would be appreciated.