Someone offers to test your tap water at the kitchen sink for free. It sounds like a fair deal, and sometimes it is. The test is only the first step, though. Before you let anyone run it, or act on what they tell you, check the points below.
Ask what the meter actually measures
Handheld meters and test strips usually read a few things, such as hardness, chlorine, or total dissolved solids. Those readings give a rough picture, but they do not show whether the water contains lead, bacteria, nitrate, or other contaminants that need a certified laboratory. Ask the tester to name each parameter on the printout and the method used. If the list is short, ask what the test leaves out and whether a lab test would cover it.
Get the raw numbers in writing
A result that says the water is poor or needs treatment is a conclusion, not a measurement. Ask for each reading with its unit, such as milligrams per liter or grains per gallon, along with the reference level it is being compared to. Keep a copy. If you do not have the numbers, you cannot check them against another source later, and you cannot compare two offers side by side.
Find out what the visit is tied to
Many free tests are the opening step of a sales call. Before the test starts, ask whether the appointment requires you to buy anything, whether you will be asked to sign anything, and whether the representative earns a commission on equipment sold. Asking directly is reasonable, and a straight answer is itself useful information. If you feel pushed to decide the same day, say you need time to review the numbers.
Confirm who is doing the test
Ask which company the tester works for and whether that company will put its name on the written results. Ask how the meter was calibrated and when it was last checked. You do not need to become an expert, but the person should be able to explain the method in plain terms without hedging.
Check the result against an outside source
Before you decide anything, compare the result with something outside the sales visit. A certified laboratory report, your water utility's consumer confidence report, or a published rating for your area gives you a second reference point. The county ratings on St. Charles Water Ratings show how local water treatment is graded, and the grading method page explains what each letter is based on. Treat that as a cross-check, not a replacement for testing your own tap when you have a specific concern.
Decide what the result justifies
A free test can justify a lab test. It rarely justifies buying equipment on the spot. Write down what was measured, the numbers, and what the seller recommends. Then ask whether the recommendation fits those specific readings or is a general product pitch. If the answer is unclear, get a second opinion before you sign anything.