This is important as the rot almost inevitably starts at the top, but in my experience it is rare for an underling to badmouth their boss to someone who they don't know (I say this as someone who should have, but didn't due to fear of being fired). Instead you have to eek out a general vibe by asking less pointed questions like:

* How often do you interact with the CEO/insert title?

* How comfortable are you with the level of transparency and your understanding of the company's overall health?

* Do you have input into wider strategic decisions or is that handled by a smaller group?

* What does the X year roadmap/vision for the company look like?

* How does senior management interact with the team? Is everyone together during the day so it's a natural flow or is the management team in their office/meetings most of the time?

etc, etc. Ask enough of these types of questions and eventually you'll wear through the polish and get a better feel for how people really think about the boss.