Most installer comparisons start with the quote. Verification should come first, because a quote from a company that cannot show proof of its credentials is hard to compare with anything else.
Licensing
Some cities and states require a plumbing or contractor license for work that connects to the water supply or drain. Requirements differ by location, so check with your local licensing board or building department rather than relying on what the installer tells you. Ask for the license number and look it up yourself on the issuing agency's public record.
Insurance
Ask for a certificate of liability insurance and a workers' compensation certificate if the installer uses employees. Ask the insurer to send the certificate directly to you, or confirm it through the insurer's own system. A certificate forwarded by the installer alone is weaker evidence. Check that the business name on the certificate matches the name on the proposal and the invoice.
Manufacturer training
If the system uses a specific brand, ask whether the installer holds training or authorization from that manufacturer, and what it covers. Training that applies to the exact model you are buying matters more than general plumbing experience. Ask how the installer handles warranty claims on that brand, because the answer shows how much support you will get after the job.
Permits
Ask whether the installer pulls permits for the work in your jurisdiction and whether the quote includes permit fees. An installer who says permits are unnecessary for your project should explain why in writing, with the local rule they are relying on.
References and complaints
Ask for two or three recent customers with similar systems and call them. Ask whether the installer returned to fix problems and how long that took. Check complaint records with your state or county consumer office and the local business bureau, and read reviews with attention to repeated complaints about the same issue.
What to do with the results
Put each verified item in a simple checklist next to the quotes. A bid that clears every check is easier to compare on price, scope, and warranty. For how we weigh these factors when matching providers, see the methodology page.