The same way we know about lots of products, by the company and/or third parties testing them? The same way we "know" how much of a drug is in a pill bought fro

The same way we know about lots of products, by the company and/or third parties testing them? The same way we "know" how much of a drug is in a pill bought from a pharmacy, or the % of a chemical in a bottle of cleaning stuff, or.... Obviously there can be cases of companies being misleading or completely fraudulent. But generally speaking you aren't testing every cosmetic, food, clothing etc. to be sure that they're made the way they claim, why would water filters be different? Legislate certification for them if needed, rather than worry about how hard it is for a single person to test at home.