How do we know that something is true?
Other than the fact that "knowing" and "true" are topics that can take volumes to discuss - that's a fairly simple thing. ... just joking people ...
The simple answer is, we really don't "know" anything with absolute certainty. We **can** personally have a high confidence that some things are the way we think they are - but that is still more of an emotional state (we "feel" certain) than it is a statement than is testable and will yield the results that we expect.
Therein lies the problem - as human beings, we "know" things based on a combination of personal experience, reasoning, education and sometimes, less formal avenues.
Personal experience is reliable to a point. We know that we "stick" to the ground - more or less - that tells us gravity is real. We know that when we walk through a door, there is something on the other side. We know that night comes at the end of day, and day at the end of night. We know not to run with scissors.
Personal experience is less reliable because we can't experience absolutely everything - so we personally can never have "complete" knowledge, if such a thing is even possible.We **can** however, become really good at some things. A carpenter with several thousand hours of experience is better at carpentry than the average man on the street. A carpenter with ten thousand hours or more of experience might be thought of as having mastery of his trade. The same holds true of plumbing, and car mechanics, and machine operation, and guitar playing. You can become good at a few things, really good at fewer things, and if you have an aptitude, you may become a master at something. Eric Clapton is a great guitarist, but he started as a schoolboy - spent hour after hour, day after day, year after year honing his craft. He's probably not worth a shit at plumbing, but he **knows** how to play a guitar.
That raises another aspect of knowledge - there is practical knowledge - "how" to play a guitar, and theoretical knowledge - "that" a guitar produces sounds based on string vibration and acoustic resonance. So "knowledge how" and "knowledge that" are two major forms of knowing - although others have been proposed.
Plato famously defined knowledge as "justified true belief". Just that statement raises the possibility that a person can believe something, but that something to not be true. That then devolves into how is something proven to be true.
For most of us, then, we know things that we have personal experience of, and we believe things with certainty, but with varying levels of real (formal) knowledge. Let me report the existence of the invisible fire-breathing dragon in my garage. I might not be able to prove this assertion - but I might be able to convince someone that I do have a dragon in my garage. What allows me to do this (assuming that someone really believed what I told them)?
I might be believable to my fellow invisible-fire-breathing-dragon believers because I'm a sober, reliable person who has never told them a falsehood. No matter that this is a real whopper, someone might actually believe this because I'm reliable. I might also be believed because that person respects me - I may have earned their respect through completely unrelated avenues, but this respect (and trust) might then transfer to accepting my assertion of a dragon infestation. My fellow invisible-fire-breathing-dragon believers might also believe my claim because they're pre-disposed to belief in dragons, and this confirms that belief. They might also believe my dragon claim because they're gullible, uncritical, lazy, not very smart - it starts to sound like I'm being critical of people, so I'll stop. My point is that there are many reasons that we believe things that aren't provable, and (I assume, from my limited experience) fewer ways to believe things that **are** provably true.
From here, we can take the fork in the road that explores ways we can be fooled (if we fall prey to verbal tricks and logical fallacies); we can take the fork that leads to how we can convince large segments of the population that things are to be believed (with and without justification); and we can take other forks into scientific method, probability, epistemology, marketing - it's a trip that could take a while.
I'll close with the idea that personal knowledge is those experiences and beliefs that serve you reliably in your day-to-day life, without the need to rigorously test their truth value. Job One is getting through the day successfully - the rest is the truly fun stuff!