By studying it--developing hypotheses, figuring out how observations would differ depending on which hypothesis is true, and then observing or experimenting (so we can observe what we need to) to do the test.

Repeat, over and over. Record what you learn.

It works really really well. That you're posting on Medium at all is a testament to how mindbogglingly well.

We do what we do on the basis of knowing things. This is why, for instance, we get vaccinated instead of, I don't know, inhaling bleach when a pandemic rolls around. It's why we are careful about our intake of simple carbohydrates when we have metabolic syndrome ("type 2 diabetes"). It's why we have cognitive behavioral therapy instead of exorcisms.

So my questions all stand.