Some guidelines may help.
1. References, contact info on existing clients (an SEO that has no existing clients to contact should be avoided). Clients can say far more about an SEO than the SEO themselves. Call them.
2. Always ask the SEO - what SEO the company would refer you to if not them (none -- usually means lack of professionalism) most SEOs can appreciate how good they really are, and where they stand compare to others.
3. Ranked results does not necessarily make a good SEO - ask specifics, what their best project for visitation rate, as well as their worse.
4. Non-disclosure agreement - Some SEOs do have non-disclosure agreements, however this is their loss to establishing integrity and credibility not your lose to protect yourself.