The problem with that is…
This rock keeps away tigers.
How do you know that?
Do you see any tigers?
I want to buy your rock!!
In my world it’s more:
I positively identified the mildew was there and I wiped it out once and for all.
How do you know that?
I stopped treating it and the mildew never came back. Further more, microscopic imaging revealed no mildew was visible. In fact, you can pcr test for it if you want.