If you need plumbing services, even if you have a plumbing emergency, always get at least one reliable, trustworthy referral. See if the company is listed on any trusted local contractor referral services. Call your local building and zoning department and ask if it knows anything about the plumbing company. Call the company and ask if it will give you recent references to contact, and call them to ask about their experience. If you don't have an ideal referral or first-hand knowledge of a company's expertise, quality and trustworthiness, ask trusted family, friends and co-workers if they've hired a plumber within the past year, and what their experience was with that company. Ask what work was done, how long it took, what it cost, what contracts were involved, and how they felt about the company, while taking notes. Then do your homework and ask the Better Business Bureau about the company's complaint records.
Ask what kind of insurance the plumbing company has, ask for a copy of their policies and bonds and call the insurance company to verify the policies. Compare the insurance to the size and estimated costs of the work you need done. Do not deal with plumbing companies who won't provide verifiable insurance. Call the company that provides your homeowner's insurance and ask what kind of contractor work is covered under your policy and what additional a contractor would need for a job.
Ask if the company provides written estimates. Get at least three written estimates of the work you need, from different plumbing companies. If there are big differences between the estimates, ask why. If you don't feel comfortable with the estimates, get three more from other companies. Don't deal with companies that won't provide an inspection of the job and a written estimate that includes time to schedule and complete work. Keep copies of all estimates in a project folder or binder for future reference.
Ask to see a sample contract before you sign or pay for anything to get an understanding of how the company does business. If you decide to work with a contractor after doing your homework and getting a written estimate, ask for a contract that allows partial payment or deposit at the beginning of the job and the balance upon completion of the work and a building inspection for adherence to local zoning and building codes. Review the contract against the written estimate and ask about any discrepancies between the two. Have a lawyer or a local building and zoning officer review the contract if possible before signing it.