How do you know your Windows-running PC is running a legitimately-licensed copy of Windows? It seems you can’t: <https://www.zdnet.com/article/is-your-windows-license-legal-should-you-even-care/>. The article says “You almost certainly have a valid Windows license if you acquired it in ...” and goes on to list the usual ways that users obtain their copies. But note that “almost”: there is nothing you end up with -- no document or artifact or anything else -- that definitively says your copy of Windows is genuine. The final decision always goes back to Microsoft: if they decide not to sue you, then your copy is probably legit.