First, check their online reviews to see if they have similar complaints. Second, ask for a meeting to review year-over-year traffic, conversions, and monthly deliverables. SEO takes time to show quantifiable results, so being patient and also having realistic expectations is important. And third, if you still feel like something isn't right, request a proposal from another agency. Keep in mind, all SEO agencies know how to pick apart a competitor's work -- just to make a sale. This doesn't always mean they’ll do a better job in the long run.