There's lots of competition in this field so take your time to choose the right person or agency for you.
I think a good place to start is by requesting a business development consultant's resume. They may not be willing to include exact details of all their work, for confidentiality reasons, but at least you'll have an idea of their experience.
Things to look for include: previous work in a field similar to your business area, memberships of relevant trade organisations or seminars attended, and perhaps reference to the range of personal contacts they have.
When you meet for the first time you need to be sure that the business development strategies this person comes up with are in line with who you are and what you are doing.
I've worked as an economic development consultant for several years, and any potential client who asks for these things from me gets extra kudos for thinking in the right way.