Do not limit your interview to your potential advisor. Ask to speak to members of the program or lab, especially the other postdocs, as they will usually be honest and tell you what it is really like to work for their advisor.
How much time does your potential advisor have, to meet with students and postdocs?
What are the expectations regarding funding? Some advisors will want you to get your own within a year or you may be asked to leave.
How involved will your potential advisor be with you and your project?
Pay attention to the lab dynamics. Do people seem to get along with each other? Is there competition between lab members on projects?
How big is the program or lab? What is the ratio of postdocs to graduate students? Consider if this is important.
Are there opportunities for collaboration with other labs or researchers?
What are the core facilities like? It is important to consider this if you may be doing more complex work.
Attending conferences and presenting your research, will there be ample opportunities and support?
Consider the geographic area where you will reside. What is the cost of living relative to your salary? Should you consider purchasing a home?