Trevor talks about which questions to ask when you look for an SEO company. Here are some examples:
Can you show me examples of your previous work and share some success stories?
Do you follow the Google Webmaster Guidelines?
Do you offer any online marketing services or advice to complement your organic search business?
What kind of results do you expect to see, and in what timeframe? How do you measure your success?
What’s your experience in my industry?
What’s your experience in my country/city?
What’s your experience developing international sites?
What are your most important SEO techniques?
How long have you been in business?
How can I expect to communicate with you? Will you share with me all the changes you make to my site, and provide detailed information about your recommendations and the reasoning behind them?