Truth has long been an uncertain thing, only perhaps people didn't know that at some time. There's a saying that "history is written by the victors", and that's not even quite true. The Internet has led to an information revolution, and so matters of truth and trust are all the murkier. Practically, some people recommend consuming multiple sources of different political leanings and believing the conserved details. That only goes so far, though; I doubt there's a silver bullet. Be cautious, and wary of your own moral axioms and cognitive biases, I guess.