As with any employee, you are going to want to find out the past type of work that they have done.
During the interview, inquire about the companies and individuals that the consultant has worked for. A mistake that people make when they see big name corporations on the persons resume is to assume that they are great because they worked for a big name company.
The thing to remember about big name corporations is that they sometimes hire consultants for short periods of time if they have expertise that is needed for a certain project.
Dont take a chance and hire someone simply because they have an impressive resume. Anyone can write down names and companies on a piece of paper.
Take the time to get in contact with the referrals and find out what exactly the person did for the person or company.
Its always a good idea to check out the referrals that a prospective consultant has given you and make sure that the claims that they make are well founded.
You want to know that the consultant can do what they claim to be able to do, and that they will be able to do the work that they will be given.
After all, you are putting your company...