Experience. How long has the contractor been in business? This trade can have high turnover. Make sure you hire someone who has been operating for two years or longer.
Preparation work. What kind of preparation work does the contractor do? A cheaper job will skimp on the prep work to cut corners. A good painting contractor will take the time to do everything that should be done, what they have stated they will do and what you both agreed would be done. The workmanship will show in the end, good or bad.
References. Can your contractor provide them? Call references before making a hiring decision.
Knowledge. Can the contractor make recommendations on what materials will work best for your project? A professional keeps up with the latest products and techniques. They will have suggestions for colors and finishes and be knowledgeable about the latest trends. Experience counts.
Guarantee. Does you contractor offer any? Besides the guarantee offered on the products by the paint manufacturers, most reputable painting contractors will offer a warranty on the application/preparation process. If so, you should expect a written one.
Formal estimate. Make sure the contractor puts the scope of the project, materials they will use and amount of prep work in a written contract, not on the back of business card or a haphazardly drawn-up dollar figure on a piece of paper. A professional painting contractor provides their clients with written contracts and specifications.
Cost. Price is naturally a concern. Getting the lowest price should not be your first or only consideration. You want to get someone with skill, experience, who is properly licensed and insured, aware of safety issues and is a good worker. That will not likely be the individual with the lowest bid. It does not need to be the highest bid, either. Just make sure not to hire on price. Look at the whole picture. Painting contractors operate on very tight profit margins. You will hear of homeowners who hired college students, unlicensed individuals and teachers out of work in the summer and were happy with the work. If the price seems too good to be true, however, it probably is.