You've already received great answers, so I'll keep it brief:
While things like references and examples of other branding projects are important, the most important variable is "Chemistry."
When interviewing potential firms, just make sure you interview the "point person," because this is the person you will be communicating with for the next 6 months, exploring with, laughing with, having complex conversations with etc.
It is incredibly rewarding to create a great product with people you genuinely like and respect, so interview that "point person." If you're lucky your "point person" will be the firm's creative director and if you hit if off, you'll have a wonderful brand development experience.
And btw, since you're asking these questions and receiving such thoughtful feedback, my prediction is that you're going to do a great job!
P.S. If you need the name of the agency I used, feel free to reach out anytime: email@example.com