We weren't there when ancient Egyptians built their pyramids; nor were we there when Columbus set foot in America. We have texts, artifacts, and other materials from the time period and that's it. How can we understand ancient people's customs, culture and behaviour with that limited "evidence"? Can you please explain how historians know the things they do about past societies?
This is an obnoxious – err, I mean great – question. The reason why we know history is correct is because people like you – often young and obnoxious – err, I mean imaginative and inquisitive – ask critical, fundamental questions about our assumptions about the past.
The dirty little secret about the discipline of history is that our understanding of the past is not “correct.” It is constantly evolving, and because of obnoxious – err, I mean inquisitive – questions and the inventive minds that pose them, the narrative is constantly improving.
It would be great if the discipline of history were in the business of writing the final word on each subject so we could congratulate ourselves for building an ever-growing library of tomes that were the final word on every aspect of the past. But that is not how it works, and when history runs out of questions (which it never does) other disciplines dive in and mix up the topic even more.
In 1998, I wrote the definitive history of Virginia City and the Comstock Lode in the American West. This replaced the previous definitive history published in 1883. So much for definitive histories!
In fact, in 2012, I offered a different slant on my earlier work with a new perspective from an archaeologist’s viewpoint – asking and answering different questions than I had considered in 2012. The point of view of the archaeologist is particularly important for the ancient subjects you raise, by the way.
As if that weren’t enough, I have just submitted a manuscript that I have titled, Monumental Lies: Early Nevada Folklore of the Wild West, recasting the past, now from the perspective of a folklorist.
Which, then, will be definitive? None really. And as it is, I now wait for some punk-ass graduate student – err I mean, a talented, inventive young mind – to cast aside everything I have written with a new perspective and better information.
I wait and expect that new volume, because that is how history works. I did the best I could, and now I prepare to exit from the stage. Honestly, I hope it doesn’t take 115 years for the next definitive history of my subject to be published. History does best when it consistently moves forward and improves its perspective.
In addition to the great answer you've already received, here's an older one from a similar question answered by u/DanKensington here:
Texts and artifacts, inscriptions, and archaeological evidence and in the case of, say, the Egyptians, actual mummified bodies is a LOT. If you're just asking how do people know it's true, well they don't take these things at face value, which is what that answer addressed. Figuring out what's true, who's lying/exaggerating, why are they lying, etc seems like half the work. A good example is Darius' explanation of how he became king. I think most modern historians recognize his story is....hard to believe at best, and nonsense at worst.
Even though it's a lie Darius saw fit to chisel his explanation into a massive rock wall in three languages. And the fact that he did that says a lot about Achaemenid royal ideology (it was seemingly important that he tie himself to Cyrus, even going so far as to marry any woman who could claim descent from him, clearly that dynastic line was very important), and though the particulars can't be trusted parts of his account are corroborated elsewhere - documents in Babylon refer to the brief reign of a king who Darius names as a rebel for example (though since Darius may not have had a legitimate claim to the throne himself calling the king a rebel is probably not exactly accurate or fair). So the things historians know and don't know combine to make, in my mind, a pretty compelling mystery that slowly gets chipped away at, as new evidence is found and old evidence reexamined.