18 questions you must ask when selecting an SEO agency
If you have a good understanding of SEO, that will help you list requirements and the criteria that an agency must meet. If you don't, then whilst it makes it harder, you can still evaluate service providers if you focus on their understanding of the basics of SEO.
With regards to assessing potential SEO providers, areas you should look for:
Their own website
Question 1. How well optimised is their own website for relevant keywords searches?
Question 2. How effective are they in using social media to communicate with stakeholders?
Question 3. What is the quality of content on their website?
Why ask these questions? Because these 3 strands are crucial to SEO and if they can't be bothered to get their website in order, then you need to push them for reasons. Granted small agencies often let their websites suffer to focus on client delivery and priorities and it won't be possible for them to compete with larger agencies. The quality of their thinking and approach should be clear though.
Their technical knowledge
Question 4. What is their critique of how your website is currently structured to support SEO i.e. is it primed to allow search engines to index webpages?
Question 5. What is their approach to duplicate content?
Question 6. How do they use Webmaster Tools to inform the SEO program?
Question 7. What is their experience with sitemaps and product feeds?
Question 8. How will they make the most of user generated content inc customer reviews to get SEO benefit?
These are examples of specific questions that will show their approach. What's important is how transparent they are with the way they respond to these questions. Particularly true when it comes to link-building...
Their SEO focus and marketing skills
Question 9. What is their approach to keyword research and keyword selection strategy?
Question 10. What does link building mean to them? How do they go about it?
Question 11. What's their approach to creating a content strategy
Question 12. What experience do they have with social media and crowdsourcing esp in relation to seeding content and outreach?
For these, you need to see how good they will be at applying these skills through understanding your business and market. Once you have a shortlist of 3 SEOs ask them to evaluate the effectiveness and method to improve the ranking for a single product/theme for your site. You will often find agencies have experience in your sector and this is usually beneficial.
Their analytics skills
Question 12. What analytics tools do they use to measure SEO impact?
Question 13. What KPIs would they benchmark current performance against to demonstrate impact of their work?
Question 14. What testing would they do to try to improve SERPs visibility and click through rates?
This is an area close to my heart - I believe use of analytics is essential to improve results from SEO - see my series on website optimisation for the way I approach this. This is really about process - ask them to explain theirs. Here's a good SEO improvement process shared by Universal McCann.
Their industry knowledge
Question 15. How do they think SEO has evolved in the past 3 years
Question 16. What major Google changes do they think are relevant and what are the implications to your website?
Question 18. Where is the industry going and what are the priority focus areas for good SEO?
Finally, or some would say the first thing to look at, consider their clients and success stories - ask to speak to someone direct at a client. Even considering picking the client you want to speak to as most reference sites are set up to give glowing endorsements as agencies don't pick annoyed clients to do this.
The one thing I haven't covered (I think) is the "how much will it cost" question - which should be asked initially to filter out those that are just too expensive for your business. See the recent 2012 agency costs research from Dan Barker to see how costs and payment models compare.
Then it will again be a factor at the end of the selection, but only after all these other questions have been answered satisfactorily.