If everything we learn are based on the books we have today (or history Channel), how can we be certain they are telling the truth, when in most cases "History is always written by the winners". Not saying they lie, just that how are they sure that's what happened.
How do people find new evidences?
And sometimes, we encounter both opposites ideas of what happened. How can we be sure who is telling the truth?
Like the Spanish American War. In the US is taught that Spain attacked their boats therefore the US supported Cuba for the independence. However, in Spain, is taught that the US wanted Cuba for their own benefit so they did a false flag, exploding their own ships dressed as spaniards.
How can we know the truth?
History is always written by the winners
It isn't. Most prominently in the Anglosphere, the Lost Cause still afflicts American Civil War history. Fuchida Mitsuo, despite having been on the losing side of the Battle of Midway, managed to single-handedly dictate its history up until Shattered Sword. For obvious reasons, the Anglophone scholarship on the Vietnam War has been largely dominated by American writers.
And of course, there is the tiny niggling matter of Philippine history, because we have been notably on the losing end of things (colonised twice, occupied three times, invaded four times!), and yet there is still such a thing as Filipino history.
How do people find new evidences?
New perspectives, new sources, translations, and doubtless others that I haven't covered. Sometimes it's not so much a matter of new sources as it is unexamined sources. Fuchida Mitsuo could never have told his lies so well if Japanese carrier doctrine was better-known in US naval history circles, for instance.
How can we know the truth?
We can't. Objective 'truth' is not a goal of history.
People lie. People lie verbally, people lie on text, and people even lie in stone carvings. (What, you think the Behistun Inscription is 100% true? If so, I've got a bridge in Minecraft I'd love to sell you.)
Bias is introduced in all levels of historical study. The sources themselves are biased. The people who wrote them? Biased. And the historians who study them are biased, and so are the other historians who study and argue against the first set.
History deals with humans. History is created by humans, written by humans, written for humans, studied by humans, interpreted by humans, read by humans.
The problem here is that the human is a stupid, selfish, blinkered creature with entirely too many prejudices, preconceptions, and biases, and a very sharply limited point of view. It is perfectly possible for a human to give a completely inaccurate picture of what is going on without even lying. Historians know this. It's part of the job. It's one of the hazards that the trade deals with, much like restaurant back-of-house deals with the hazards of knives and stoves.
Fortunately, there is such a thing as the historical method, the same way as there is a scientific method. Here are some previous threads for you to consider:
Also, see next post.