SEO Techniques Used. Ask these questions: Do they use Ethical Tactics or engage in Underworld Tactics? Make sure that they use white hat SEO. Though Black hat may get you good initial rankings, in the long run, you'll be sorry when you get sent to the back of the line when found out.

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Do they follow the Google Webmaster Guidelines? What are their most important SEO techniques implemented on their own site and clients' sites? Do they provide any online marketing services or recommendations to complement your organic search business?

Proof of Experience and Effectiveness. Ask these questions: Do they get high rankings on competitive keywords or do they only boast about the easy pickings? Can they show you examples of their previous work (SEO Portfolio) and share some success stories? What kind of results do they expect to see, and in what timeframe? How do they measure their success?

Do they give any guarantees and can they be believed? Be wary of SEOs that claim to guarantee rankings, allege a "special relationship" with Google, or advertise a "priority submit" to Google. Google doesn't have priority submit. Moreover, be wary of guarantees that are worded vaguely enough to never be called in. And beware of the guarantee that is mentioned on the marketing material or sales spiel and never heard of again on the contract page.