Check contractor or plumber license.
Before hiring contractor make sure he's properly licensed and/or registered in your state. Most states require contractors to prove their knowledge and work experience, to obtain a general liability and workers compensation insurance. A primary purpose for licensing and registering contractors is consumer protection.
Licensed contractors in many states are required to have a bond, and insurance. A bond could serve as a source of funds available to a homeowner if work is negligently performed. Liability insurance covers accidental damage to your (or your neighbor's) property caused by your contractor. Together with the worker's compensation policy it may also cover accidental injury to contractor, his employees, and others resulting from the contractor's work on your property. (Otherwise, you as the homeowner may be personally liable, since the accident happened on your property).
So, to protect yourself and your property, hire licensed contractors for your home improvement projects and repairs, particularly if the project is complex and expensive! To find how and where you can check if a contractor is licensed, click on your home state below.
Unfortunately, there are bad and dishonest contractors, which are properly licensed and registered. If you've hired a licensed contractor but are not satisfied with the job he's done, you may find how to deal with a bad licensed contractor here.