Well, for one, there is in fact a historical record here. There is a bit of wrangling on how exactly to match up the Egyptian calendar to the one we use, but we can place the construction into a reasonably tight set of dates from historical sources alone. It is not as though they were "discovered" by Europeans: the Egyptians talk about them, the Greeks talk about them, the Romans talk about them, the Arabs talk about them. It would be rather remarkable if the subject of the giant artificial mountains near consistently one of the most densely populated areas of the world was never discussed.

Beyond that, there are certain pieces of stylistic evidence: the Great Pyramids did not appear ex nihilo, there was a period of development from the even more ancient mastaba to Giza, to the Step Pyramid of Djoser to the various pyramids of Sneferu, and finally to the pyramids of Giza. And funnily enough there aren't really great stonking big pyramids after Khufu and his descendants, they are fairly characteristic of the third and fourth dynasty. There are certainly big pyramids built after but not on the same titanic scale. So on this stylistic grounds we can be pretty clear on this.

But if you want something really concrete, there are the carbon dates of material from the pyramids. Carbon dates are a bit more fickle than is usually appreciated, but the dates at least confirm that they were constructed within the commonly considered "historical period" of Egypt, and once that is conceded it requires a lot of mental gymnastics to disregard the traditional dates.