If you're interviewing at a shop who's work you're not familiar with, ask them to show you everything they've done within the past year.
Sometimes they'll already have it on the walls. Ask them about each piece - who worked on it, what the process was like, what they wish they could have done better.
When you're interviewing, don't take the agency's word for it that they're committed to doing great creative. Ask to see what they've done recently and make them prove it.