Get it documented: Once the technician diagnoses the fault ask him about it and the way to repair it. Ask him to furnish a whole document that will contain the scope of the work. It will document everything that needs to be done, the time it is going to take, and also the cost break downs. It should also contain the warranty information. If you system needs a replacement a well-trained, up-to-date contractor will not try to discourage you from purchasing high-efficiency equipment. Less trained contractor who neither have knowledge about modern technologies in cooling nor know how to install and maintain those will not encourage you to buy them.
People skill: Lastly, the technician should have people skill. You should feel comfortable with him when he works on your premises. He should be courteous. You should be able to ask him question and he should answer them amiably at the best of his capacities. The company should check in with you to ensure you are satisfied with their work, because it is their goal to create happy customers each and every time you make a service call.