I would google bunch of SEO related keywords (like "how to rank high on Google" or something like that).. see who pops up in the SERPs. If SEO people/company can't SEO themselves.. that tells you a lot about them. You don't take diet tips from overweight people. You don't take investment tips from homeless people. Don't take SEO tips from people who don't SEO themselves to get business. My 2 cents.