What makes sense to me is to ask them to describe how they handle part of the Agile process. Right now my favorite is the beginning of an iteration, but you might develop your own favorite.

Ask: "given a pile of tickets at the beginning of the sprint, describe your workflow from here"

Key points to listen for here:

• Do the devs estimate the tickets?
• Do you keep track of velocity?
• What happens when your estimates come to more than your velocity?
• What happens when your estimates come to more than your velocity when you have a deadline? (Watch for spin here: do they reduce complexity, or reprioritize, or just deathmarch the development team?)

None of these are deal breakers by themselves, but if their answers to enough of these questions make you wonder, then maybe they're interested in agile rituals, not actual agile development.