I went through a similar search last year and the hardest part was telling real capability apart from a nice looking portfolio. In the end I compared three agencies based on how clearly they described their process rather than on price alone. One of them listed custom software, website development and SEO as separate services with an actual explanation of scope for each, and before any call I always read that kind of page carefully, you can see what I mean in this link, because vague wording usually means the scope will be argued about later. I would also ask upfront how reporting works and who owns the code once the project is finished. Those two answers told me more about a team than any case study did.
