Well, the easiest way to confirm something is to look at other sources.

You are right that its possible for someone to lie. Lets say that writer 1 says that the emperor loves hats, now we can look at other writings on the emperor, and see if theres any discrepancies. If we look at writer 2s works, we can learn that the emperor once spent a lot of is money on hats, and why would a man who doesnt like hats spend so much on them?

This is more of a metaphysical question as it deals with the question of "what can we know?" and "can we trust that which we observe?", and might be better served in /r/askphilosophy.

Recent historiography is actually very aware of these questions, but tend to set them aside in order to do the work of history, which can't be done at all if you're operating under a Cartesian-like "evil genius" assumption with regards to the existence of the world.

However, metaphysics is definitely the field of philosophical study that attacks that question head on, so as mentioned, you might be better off going there.