To choose a local plumbing service, ask friends, family, neighbors and coworkers about the experiences they have had with plumbers. Ask for both positive and negative reviews, so you know which companies to avoid. If you don't get any good referrals, continue your search on the Internet, using websites such as Best Plumbers.
If you have encountered a plumbing emergency, you may only have the time to look in the Yellow Pages to choose the plumber who can come the fastest. Call each plumber. Ask each one how long he has been working in the field and whether or not he has an established office. Ask if you can see some examples of the work he has done, such as at a former client's house. Ask the former customers about their impressions, and find out if there were any problems with the plumber.
Choose a plumber who has insurance. Find out if the plumber provides a guarantee for his work and what happens if the plumber goes out of business and something breaks. Try to choose a plumber who is a member of a professional trade body, as these organizations usually require members to follow specific rules and intervene if a problem with a client arises. Obtain a price estimate before hiring a plumber.