How do I know my plumber has a valid license?

In Massachusetts, the license number must be displayed on all advertisements because Mass General Law (Chapter 142) states in part: any sign, listing, or advertisement of a Master Plumber, Master Gas Fitter, a Journeyman Plumber or a Journeyman Gas Fitter shall contain his designation and license. This includes websites, phone book advertisements, business cards, truck lettering, and more. By visiting the website of the Division of Professional Licensure, you can check a license very easily.  Select the highlighted link and then choose “Check a License” on the lower right hand side of the page. Even if you don’t have the license number, you can check by individual or business name. This will show you the license number and the name of the individual who holds the license. Easy.