Go with your gut. This might sound like odd advice, but if you're uncomfortable speaking to a potential SEO Agency then it's probably because you don't understand something they are telling you. SEO is a highly technical and complex subject, so if the agency you are planning to work with can't make themselves understood to you before you even start working with them, then you might not have a great experience with them. Clarity is key here; ask challenging questions and don't accept any answers until you fully understand them. SEO is complex, but every single bit of SEO theory relates back to something in the 'real world', and that's where you and your clients or customers live.