Facebook, Facebook, Facebook. Any reputable shop should be on facebook sharing their services and their instructors to connect with customers. You will be able to easily contact these users for their experience with the instructor. Lack of Transparency and public knowledge should be a red flag that an instructor is not that well liked. If the shop isn’t Internet inclined then ask for contact info for a few customers and get their opinion. Dive shops live or die on their instructors abilities so a shop not willing to give references should be avoided.