Even if there were only four people who had ever possessed clairaudience, clairvoyance, necromancy, telepathy, or any of the other mystical abilities stroke psychic powers, that would be proof that they exist(ed). You don't get to take a mean average to see whether a manifestation is real. The test is whether whatever is predicted, scryed, and so forth, could not have been arrived at by some other means.
And this is where the sceptics place all their emphasis. I would rather observe that powers possessed by 'dumb animals' would make us appear extraordinary if we had anything like them naturally within us. I'd also note that it is true that you are more sensitive to experiencing or observing events if you still your mind and allow it to focus on something other than the everyday.
Claim to possessing a heightened sense can be tested easily enough, but it would have to be in the form of observing the 'gifted one' working in the field. You would simply damp what power they have by placing clinical conditions on their validation. A lost child or a solved mystery will do splendidly.