You'll want to take a look at a few things.
One, take a look at their past work. You'll want to see their portfolio and better understand what specific issues they helped their clients solve. Was it a rebrand? Did they create this new product from scratch? Were they entering a new market? Branding isn't only about a story, but the right one for the right audience. And are you like those past customers (i.e. are your needs similar)?
You'll also want to see that you like their aesthetic. Does their work resonate with you and do you feel inspired? If not, then why would your customers?
You'll also want to know how they source talent. Are they bringing on subcontractors or is it their firm you'll be working directly with? And what is the payment structure? Are you paying a retainer or a project fee or hourly (which I wouldn't advise any firm to do, but they might)?
These are just some of the questions you might want to consider. Mainly, it comes down to do you like their work, are they credible and can you rely on them to get the job done?
Let me know if you have any other questions. Happy to help you review some candidates over the phone. All the best!