Branding consultations are -- or should be -- conversations.
You can identify talent based on a firm's past work. But you may find midway through an expensive branding initiative that your business and your branding consultant(s) miscommunicate like an ill matched couple.
My advice would be to proceed incrementally.
Talk to a few people for 10 minutes each. Look for 3 things:
(1) Credible expertise
(3) Value for time / Value for dollar
If you reach a dead end or realize that you're not getting real value for what you're putting in, let it be after 15 minutes or $50 instead of after 3 weeks and $5,000!