The internet doesn't solve the problems that you seem to think that it does. People can lie on the internet. That said, when you find a piece of evidence, you don't just take it at face value, you cross-reference it with other sources that you think may be related. So, if you find an ancient tome, you can't say that it is accurate until you find out if its contents agree with other sources that you think are reliable because they, too, agree with other sources.

Because we don't have time travel, we don't "know" anything for sure about history, but we can get a pretty damn good idea about it from cross-referencing different sources to compile a more complete picture.