The first and quickest way to vet a firm/freelancer is by looking at how they represent themselves. We can get an initial sense of quality by visiting the website of a firm and doing a quick analysis.
The last question is perhaps the most important one. Especially if you're looking for a development partner to help build your website or web app (read about the difference here), you'll want to make sure their website meets your standards. If they can't be bothered to make their own site more than good enough, you can't expect them to give your site the attention it deserves.
Running a business is hard,
Software development shouldn't be ✌️