In my experience small SEO agencies sometimes have a more personal approach. Maybe it's a good idea to invite 2-3 agencies and you begin straight away with your questions. I have found myself in the same spot with a company who didn't want to tell about their methods and 3-6 months later they went out of business because they bought old authoritative domains and transformed them into blog networks/linkwheels. Nowadays when we want to hire a certain company we go to smaller agencies and tell them we don't want bla bla but easy answers in normal language and feedback. Good luck with your business.
