Most homeowners compare quotes on price and then maybe skim a few reviews. Before you put two bids side by side, confirm that each company is who it says it is, carries the right insurance, and can show the kind of work it is pricing. These checks take less time than a single service call, and they can keep you from choosing a low bid from a company that is not set up to do the job.
Ask for the license number in writing
Local rules differ by city and county across greater St. Louis, and some installation and plumbing work requires a license from the municipality where the house sits, not only a state license. Ask which license applies to the work in your quote, ask for the license number, and check it with the issuing agency's public lookup if one is available. Confirm the license is current and that the name on it matches the company name on the quote. A license held by one owner while a different company sends the crew is a mismatch worth asking about.
Get the insurance certificate, not a verbal yes
Ask for a certificate of liability insurance and, separately, proof of workers' compensation coverage for the crew. Check three things on the certificate: the named insured matches the quote, the policy is active, and the coverage limits are listed. If the company says its crew is covered under someone else's policy, ask for that certificate too. Gaps in coverage are easy to miss when nobody reads the paperwork, so put the request in writing before you schedule anything.
Ask for work you can check
A reference list helps only if the company can tell you exactly who did the work. Ask for installations of the same type you are buying, completed in your area, and ask whether the people who would visit your house are the same people who did those jobs. If they are different, ask how they were trained and who supervises them on site.
Check the product listing behind the claim
If the quote names a specific filter, membrane, or UV unit, ask whether it carries a third-party certification listing for the exact claim being sold. Common listings come from NSF or the Water Quality Association, and each one covers particular contaminants under particular conditions. Ask for the listing number, look it up yourself, and confirm the claim on the quote matches what the listing actually covers. A marketing line about purity is not the same as a listed performance claim.
Put the answers side by side
Build a simple sheet for each bid with five lines: license number and issuing agency, insurance certificate status, who will do the work, product listing number and what it covers, and the company's written description of its warranty. Fill in the same five lines for every bid. A bid with blanks is telling you something, even when its price is the lowest.
If you want to see how the Best Water Companies STL awards weigh these kinds of factors, the Award Methodology: How Winners Are Chosen page explains the process.