If you don’t know what you are really looking for, you will get ripped off. So the first rule is to research the product or service you are looking for. If you are looking for a roofing contractor, first research about the roofing work, process, material, costs and etc. Then you should get quotations from several contractors. You should compare several quotations to get an idea about the process and cost. Do a proper research on the company you are planning to hire. Reviews are very important when hiring a contractor. Get genuine reviews.There are review sites like Angie's List
with real user reviews. Please remember that you should not pay the whole amount until job is completed.