Choosing a SEO company is a big decision. Why? A SEO Company can improve your site ranking, do nothing or even do damage to your site and reputation.
Questions to as the SEO Company:
Examples of your previous SEO work (success stories)?
What kind of measurable results to expect, and in what timeframe?
What are the exact and total costs?
A number of Marketing Institutes and Marketing Schools will also have courses that will give you the general knowledge to be able to find hte SEO Company that will be the best match for your needs.
There are also companies that giving presentation that will train you on how to choose a SEO Company. Here is the promotional vide from John on Outsourcing SEO Projects. It is a good start in searching for your SEO Company:
Senior Manager of Emerging Media Marketing