If you don't want to make them defensive, I've found the following question will initiate a conversation that will tell you everything you need to know about whether they are actually using an Agile approach or just paying it lip service:

Who is responsible for writing requirements/specifications for your software projects?

I have seen numerous companies who claimed to be agile and even wanted a Scrum Master certification describe a classic big up-front design process when you ask about their requirements gathering process.