History always be rewritten by the country government. Books that write against government usually be burn and destroy. People go against government usually be hunt down as witch. How could historian able to tell the truth?
There are a number of ways, but one of the largest (in my opinion) is the bias for why it is written. Was it written as a report or as propaganda? Was it written by a random guy who read about it in the newspaper or an eyewitness or participant to the event?
We also look at consistency with sources over reports. If 10 separate sources say something did happen, and 1 says it doesn't then you would edge towards the majority, however you would also look into WHY that one source says it doesn't.
The science of history is a very large dose of skepticism mixed with tenacity and research.