1. Make Sure They are Licensed in your state and town
2. Go with a company that charges a flat fee for each service. These companies know their business and the amount of time that each problem should take to fix. The cost may seem higher up-front. However, you will not get any hidden surprises.
3. Insured. Make sure they have the proper insurance that is required by your state and town. The licensing usually requires insurance. It is still a good idea to check and make sure.
4. Read online reviews from sites like Yelp, Angie's List, BBB online, or call your local BBB.
5. Neighbor reference.
6. Length of time in business. A plumber that has been in business for more than 10 years is a huge plus. The plumbing business can be difficult and the ones that have lasted usually provide great service.
7. Clean Business and Trucks. Plumbing can be a dirty job and that job can make your house a mess. The plumber with the clean truck and building has pride and he will take that same pride into your home. To be sure ask the company what steps they take to protect your home from dirt.
8. A warranty of 6 months to 1 Year.
9. One that answers the phone or has someone to answer the phone.
10. 24 Hour Emergency Phone Number and Service.