Facts and reliable sources.

I can determine through independent study that the holocaust happened. There are still living survivors to it, the camps and associated artifacts are preserved, and there are numerous contemporary sources that independently verify the events.

Moving further back to an event where there are no living witnesses, let us consider the US Civil War. Same thing, I know it happened, because there are mountains of evidence, when looked at through a critical eye establish that the war happened. I can see newspapers and again preserved accounts of the event. I can look at those accounts, and other evidence of the passing of armies and governments and reach my own conclusions, and find that those conclusions agree with others who have done the same or similar research.

Going even further back in time, it's the same process. Existing records, artifacts and recorded accounts of an event studied with an unbiased eye, with opinions formed based on the facts given. From there, you can determine how valid other sources and research is.

This question might be more suited for /r/askphilosophy, as it deals more with the philosophical question of "what can we possibly know?" and "can we trust whether what we see or hear is real?" As well as, "what is real?"

These issues do in fact crop up in history, as a lot of philosophical thought filters its way into historical analysis, from positivism to poststructuralism, and so on.

However in general, historians leave the bigger philosophical questions alone (like yours of how can you rely on any account) and delve into the smaller ones (like what does this account, even if partially fabricated, say about the person who said it, and say about the event's plausibility).