Tip #1: Ask about licensing and materials.
While not every state requires licensing to clean or repair duct work, there are, in fact, several which do require such certification. Be sure to check with your local BBB to see what is required in your area. Additionally, ask the company about the chemicals they use to clean the duct work. There can be some serious drawbacks for certain clients, and it is not necessary to use them for all cleaning projects.
Tip #2: Get an estimate.
Because cleaning your duct work and heating system can take some time, it is important to find out about the company's hourly rates. Be sure you ask for an hourly rate and an estimation of how long the project will take, so that you are prepared for the final cost.
Tip #3: Inspect their work.
While you do not need to follow the technician as he or she works, it is wise to look over the work that has been done before you pay and bid the technician goodbye. Look over all intake vents, blower fans, heating coils, access doors or panels, and plenums. There should be no visible dirt there, or an any of the rest of your belongings.