Unfortunately, even knowing what is false is not truly possible. Knowing that something is false is itself a kind of ’Truth’ and you’ve argued well that science cannot really do that. To know what is false, you must, at least, assume the premise that the evidence indicating falsehood is true and that it has not been tampered with. People can doubt any evidence and we often see that. In extreme cases, people can even doubt their own senses and their own experiments or they can assume that their evidence indicates something different.
Where does that leave us? That leaves us with evaluating plausibilities and neither truth nor falsehood. One of the frameworks that clarify this is Bayesian probability theory (see https://bayes.wustl.edu/) and the view that probability theory may be viewed as extended logic.
Of course, plausibility is a tricky thing. It depends on the background and the a-priori assumptions of the person doing the investigation. What scientists take as given is just the latest assumptions and views of the majority of the scientific community. In my view, what is most precious about science is its process of giving precedence to public and repeatable evidence but its theories are certainly an evolving thing. As the saying goes, ‘becoming is superior to being’.
As for conspiracy theorists, some of them may agree on the use of the scientific process. However, they simply share different assumptions or base premises than the majority. Historically, many conspiracies turned out to be true so it would be short-sighted to off-handedly dismiss all of them. And you can’t really tell which of the many ‘conspiracy theories’ might well turn out true in the future. The best you can do is to enunciate your current model of the world and identify the pieces of evidence that you trust and that support the current models you’ve accepted.
In the end, there is a lesson to learn. In the words (but not meaning) of Keats: “The best lack all conviction, while the worst Are full of passionate intensity” … I wouldn’t go that far but too much passion in asserting righteousness, even if based on the scientific process, is not well-advised.
