I was actually the only member of the IT dept in the interviews for our Director of IT position some years back. I was also the only person involved in the process that was not on the existing executive management team.
I think a lot of it depends on who else is in the room. If there are any other department heads involved, note closely their interaction with YPNB (your potential new boss). I found that to be quite telling in that you see what is important to them, and if YPNB has any intelligence, they will pick up on what is expected of them and what the others are looking for. By being involved in this, I was able to pretty much garner what projects we would be working on during the first 6 months after he was hired.
As others have stated, HR or someone else may run the show. The only thing you might want to consider is this: if YPNB requests a laptop or projector for any type of presentation, hook it up, but do something wrong (leave a plug loose etc). See how YPNB reacts to the unexpected and how the interaction goes when either they or you "fix" the problem. I don't know of any other way to really get a read, because any questions from HR (or you) are likely to be met with "correct" answers.
Also, with the boss I ended up with, we had a laptop setup and he wanted to show something off a thumb drive. At that time, the NIC port was wide enough that you could put a USB drive in it. He did. And so I went to the laptop, noticed what he did, and quickly put it in a USB port without saying anything. He showed his stuff, and ended up getting hired. He was nervous about the interview, but was very gracious about the whole thing. He is still my boss today (6 years later) and while not perfect, we could have done a lot worse. He was a unanimous choice (they even let me vote).