How trustworthy exactly all these old records sifted thru by historians? Who recorded them? Are they credible? Even the now modern societies still deals with lots of information falsification in national governments let alone regional ones, corporations let alone small ones albeit increasing intellectual demands of general public plus so many investigative journalists. I mean the whole of history, how to be sure there are no manipulation of data, negligence/incompetence of these whole history gigs. And not mentioning frequent revisions & reinterpretations of history. Are there any acceptable safeguards & what are they? Sorry for sounding childish plus making so many assertions. These are just random thoughts.
You've identified the number one issue historians grapple with on a daily basis! :) It's almost impossible to answer this, because answering it in full would just be an entire course on historical methodology, but my brief answer is that you ask those questions. You never look at a source and just say, "Oh good, so this is what happened, I'll mark that down." You always question the motives and accuracy of the writer. Where did he get his information? Does it seem plausible given what other writers say about the same thing? What were his biases and ideological commitments? Did he have a reason to lie? Could he have been wrong by accident? Is there other evidence to back up his claims? And then you bring all of that context to bear on interpreting what he said. It's a lot like being a police officer interviewing a witness. You'd never assume that a witness statement involves no manipulation or falsification just because! Instead, you'd try to make an informed and intelligent decision about the statement's accuracy based on the totality of the information you gather from all sources.