Reviews and recommendations are great, but they may not give you a complete idea of the skills and expertise of the contractor you are researching. Ask the contractor for a full list of their completed projects and what part they had in them. This will help you to understand if they've previously worked on jobs like yours and if the finished work meets your expectations.
A good rule of thumb is to request an estimate on your annual energy bills. This is a good test because experienced HVAC contractors should know the equipment they work with and be able to perform heat load/cooling load calculations accordingly.