I suggest you try to design some behavioural questions based on the attributes you're looking for in a boss. For a manager, I'd focus more on communication, leadership, supervision, organizational and delegation skills than technical ones.

Apart from interviewing him/her one-on-one, I'd further ask the applicant to give a technical whiteboard presentation to all members of your team and allow power of veto to any team member (this is what Peopleware calls an "Interview Audition"). I'd also have at least two other team members interview him/her one-on-one.

Here's a random selection of some of my favourite interview questions:

• Why did you choose your current career? Ice-breaker: lets the person talk about their past and how they got to where they are. Helps you understand them and their goals/values.
• What are you good at? Bad at? Strengths? Weaknesses? Put the candidate at ease by telling him/her about your strengths and weaknesses. Again, a good ice-breaker, hopefully allowing you to break down the barriers and know what the person is truly like.
• What motivates you? Describe a specific situation when you were very motivated to perform a task. Describe a situation when you felt unmotivated.
• What two projects are you most proud of and why?
• What are your favourite tools? When you start a new job, what tools do you immediately download and install?