Allow me to make an assertion. Nobody knows what the truth is. No one has the kind of satisfactory access to the universe which allows them to determine any kind of absolute truth. This means, in turn, that anyone who tells you they know the truth of something is at best mistaken and at worst outright lying.
The Truth vs. the truth
When we talk about things being true, it seems like what we’re really talking about is whether or not we should take things to be true. How many times do we think 
something is true only to find that there’s some contradictory evidence? I used to believe in Santa Claus, to the point where I would lay traps for him in the living room in the hope of capturing his toy sack, until I noticed that he and my mother had the same handwriting. I took it to be true, but was required to change my position based on new evidence.
We Are Already Doing This
Imagine if someone did have access to the truth, and told you what it was. It seems like one of the first questions which comes to mind is “How do you know that’s true?” You want to know my reasons for it, why I take it to be true. Even with something as controversial as a revealed truth, it’s still intelligible to ask how we know it’s true. We does this every day in our daily lives, whenever we try to get confirmation about things which are going on around us, and it’s the very essence of critical thinking.
Three Good Reasons
Here are three good reasons to believe something is true. The best propositions, such as the law of gravity, or evolution, but also more basic things like whether or not it’s raining, will have all three going for them.
Verification
The ability to independently verify, touch, handle, and see a thing from all angles. Things like physics and biology fall under this umbrella, but also archaeology, geology, and paleontology, anywhere that physical evidence is paramount. Sociological facts are also verifiable, though it can be more challenging due to their reliance on the personal experiences of others.
Consistency
It’s important for something which we take to be true to be consistent with our other evidence. Once, I believed that Santa had a workshop at the North Pole, but if I had gone there and found nothing, that would be inconsistent with my previous belief. Only one of those can be true, and not seeing the workshop would have the benefit of verification, forcing me to look for more evidence if I want to continue believing in Santa.
Inference
Some things are true in virtue of other things being true. For example, if it’s raining, then it’s cloudy. Which is to say that if we verify that it’s raining, and know that rain is consistently accompanied by clouds, then we have good reasons to infer that it’s cloudy, even though we haven’t verified that there are clouds. This is also how surprise parties work. You have no good reasons to infer that a bunch of people will be waiting at home with balloons and cake, and so you’re surprised when it happens.
Not True? Not the End of the World
We’re wrong all the time. Part of this blog is about being wrong, for that matter. We misjudge how many steps there are on a staircase, or how much time we really have to make it to work. We’re wrong about bigger things too. I used to think it was true that we only used ten percent of our brains (it’s not), and then I found evidence which demonstrated otherwise. How we react to that is up to us. We can see it as a crushing defeat because one of our favourite truths has been shattered, or as a great moment where we get closer to something resembling actual truth as we acquire better reasons to hold a new position (Alternately, we can cling obstinately to our old position in the face of any new evidence, but I’ll save that for another time).
So?
We use these reasons instinctively every day. We verify that we have our keys before we leave the house, and consistently expect our toast to pop up at approximately the same time. If we see wet sidewalk and people with collapsed umbrellas, we infer that it’s been raining. Becoming conscious of the process and applying it to our more sophisticated beliefs allows us to test them and take things to be true which are more reliable than what we had, and more reliable is usually better for us. It also gives us a window into how to deal with conflicting truths, a situation we can run across everywhere from the classroom, to the workplace, to the bar. Having a system in place to analyze reasons can allow both parties to walk away with better reasons, rather than just proclaiming their correctness at an ever-increasing volume.
Perhaps you'll think twice the next time I try to suggest that quantum science isn't necessarily the end all be all of the universe's building blocks – we used to know for a fact the earth was flat, that the universe revolves around us… who are we to ever assume that we know for a fact anything we perceive when some time later we discover a new way to measure and perceive the world around us.
That's an interesting remark Dave, and I'm glad you brought it up. Quantum mechanics and linear dynamics (the system which we used to understand quantum interactions) are particularly sticky when it comes to determining what we ought to take to be true, because our traditional notions of physics seem to get a little wonky when things get really really small, really really large, or really really fast.
First, let's briefly explore the question. Are there good reasons to take it to be true that quantum mechanics is fundamental?
Well, we can observe their interactions, after a fashion, and describe those interactions mathematically, with linear dynamics (link). Consider a phenomenon like nonlocality, where electrons affect each other at any distance without information travelling between them. Electrons are quanta, and are thus classed as fundamental particles. Particles or waves which affect them would necessarily be detected. At least, we'd like to think so. Physicists like David Bohm have posited theories of hidden variables, wherein a quantum wave guide shifts the spin of electrons (Bohm, 1952). While this explains the phenomena, there is as yet no positive evidence for the existence of a wave guide except as an explanation for nonlocality. Furthermore, Bell's Theorem seems to prove that no theory of hidden variables can maintain locality (Bell, 1964). If there are no hidden variables, then quanta are fundamental, and merely acting in ways we haven't figured out yet. This leads to questions about our state of knowledge, and the rise of Quantum-Bayesianism (Fuchs 2010). And yet, the physics works. So consistently, in fact, that we use it in consumer technology, like the ipad. For more information on linear dynamics, I recommend "Quantum Mechanics and Experience" by David Z. Albert. It's a good introduction and overview of the subject.
So we have experimental evidence and mathematical proofs which demonstrate that, at the very least, as yet there is no evidence for the presence of hidden variables in quantum mechanics. Are consistent experimental evidence and mathematical proofs good reasons to take something to be true in physics? It certainly seems so. We find them to be good reasons to accept that force=mass*acceleration, and accept them as tests for cars, airplanes, and most other inventions.
You raise the idea of historical facts, which is good. People up to Pythagoras and Aristotle assumed that the earth was flat, and they did so for good reasons. They had no evidence which contradicted that claim. Similarly with the geocentric view of the solar system. it took philosophers and astronomers like Eratosthenes and Galileo to produce evidence which confirmed their claims, demonstrating with mathematics and observation respectively that the earth is round and orbits the sun. If observation and mathematics are sufficient for believing those assertions, is there a reason that they are insufficient for accepting the findings of linear dynamics?
Now, you have a second question in there, which asks "Who are we to assume we know for a fact anything we perceive?" And that's a really good question. Let's take facts to be "Things which we take to be true for good reasons", which in light of my post doesn't seem unreasonable. It's hard to imagine that when we talk about facts we're talking about metaphysical truths rather than observational or logical ones, because as I've pointed out, we don't seem to have access to those. We take these things to be facts because they are backed by the kinds of evidence on which we already rely in our everyday lives. They can be overturned and replaced by new facts, but only by the addition of further evidence.
Consider this: you seem to assert that you believe that the earth is round. What if I assert that it is a cube, and that we will eventually find ways to measure and demonstrate that it is a cube? I decline to accept the current evidence you offer toward its sphericality, because we could find out tomorrow that it isn't true. Now, what good reasons do I have for believing that the earth is not round? I have no data, no observation, no mathematics…I have an idea that knowledge expands, but no foreknowledge about how it will do so. This position, apart from being utterly unfalsifiable, would also enable me to believe that toaster are mammals and that gravity is literally the Man keeping us down. It is, in short, untenable.
So if you have some positive evidence which supports the claim that there are hidden variables or further levels of particles beyond that of quanta, I'd be very interested in hearing about it.
I'm just trying to figure out why you insist it to be true…
"This means, in turn, that anyone who tells you they know the truth of something is at best mistaken and at worst outright lying."