The easiest one would just be to prove that the person in question didn't have the many thousands of programmer-years at their disposal necessary to make something so large and therefore the only possible source is the one entity that ever did, and used it to make Windows. Since "some guy who leaked Windows" is obviously not in possession of that sort of programmer-power, it's a pretty open-and-shut case.
That one is so obviously going to win that speculating further isn't worth much, but there's a lot of other ways we could prove it too based on architectural similarities to the existing binary code (which I presume is not wiped off the face of the Earth), but this one, while quite sufficient to prove the case as well, is more abstract and prone to internet snipers trying to prove their smarts by quibbling about endless leaves while missing the obvious location and sheer mind-blowing size of the forest. But we don't have to go here, because the previous paragraph would do just fine.