I came across a video where a website owner asked a Google employee "Which SEO Agency do you recommend? He didn't give one company as an answer but he did give some great guidelines on how to choose an SEO company. It includes what to look for. What to know about etc. The most important part of it is Google Search guidelines which I have written about before. Everyone that wants to promote their website or is looking to hire someone to do it for them should read the guidelines before interviewing anyone. The other item mentioned here, is that if a company won't tell you what they do be extremely leery. there isn't any magic in SEO most of it is common sense and hard work. You can find the articles here on this website. Here is what Matt Cutts has to say about how to choose an SEO company.