[IceMetalPunk]

Here's the thing about truth: the "who" doesn't matter. There shouldn't even be a question of "who determines if something is untrue?", nor should we be advocating for every individual to decide the truth for themselves. There is one objective reality that exists outside of our own minds, desires, and decisions, whether we like that reality or not.

What determines truth is empirical evidence. If something has no empirical evidence, it is untrue. If something has empirical evidence, it is true. If there is conflicting evidence, then some of that evidence is invalid and it must be re-analyzed using math, existing knowledge from provable things, and formal logic. After doing so, you will either determine which of those things is true, or arrive at the conclusion that there isn't enough evidence either way and stop after saying "I don't know" rather than deciding which version you prefer.

It's not about appeals to authority. Expertise is about people who have more practice at finding, testing, and analyzing the evidence in their field than random Joe Schmo on the street; and it's about nothing more than that.

We should not let people "make up their own minds" on conflicting evidence -- which is another way of saying "let people make up their own reality and expect to live in it" -- we should encourage everyone to stop at "I don't know" when they aren't sure where the evidence actually leads, and defer to people who can follow the evidence, if such a person exists. And if no such person exists, then we as a species should all stop at "I don't know (yet)".