My brother is a Moon denier. He says the proof is that we never aimed the Hubble telescope at the moon to show the flag.
Except, of course, that it's from NASA and therefore everything is a lie including "the" and "and". Even assuming that, though, anybody with an elementary knowledge of telescope optics can compute the angular resolution of an instrument the size of Hubble ... and then convert that to a GSD at the distance of Moon. Then, he could compare that GSD to the (alleged) size of the lunar landers ... and then realize that he's on a fool's errand. He's gonna need a bigger telescope.
diffraction limitation. The maximum resolution of any telescope is directly dependent on the size of the aperture. To see the flag on the moon from the earth would probably require a parabolic mirror the size of a city block or more. So it’s not really fair to say that the only way to prove we landed on the moon is to build something that is next to impossible.