It can be difficult to find an SEO agency that has your best interests in mind. More than anything, SEO rankings are about skill: a good SEO agency will keep track of the changes they make, and keep you as the business abreast of them as well. They'll understand Google and Yahoo!'s best practices, and follow them in optimizing your website. They'll do all this and more to impact your number of impressions: in other words, their job is to make a difference.
That naturally leads to one of the questions you should ask an SEO company after you figure out whether they're a broad fit for your company, in terms of affordability, location and other factors, but before you ask anything else.
That question is: "can they share some information about their past clients, and how they improved their search rankings?"
Many SEO agencies pride themselves on confidentiality, and that's an important aspect of the SEO business for their staff to know. But the best also understand that real examples of work they've done will do the most in convincing clients of not just their veracity, but that their services are worth every penny.
Whether you're looking your first SEO agency or to find a new one, make sure this question is among the ones you ask.