How do you personally tell what's true nowadays?

If I hear it from several sources, then, odds are it must be true.
Of course, as Johnston’s Law says, “Murphy’s Law is always in effect, and it can affect YOU.” (“YOU” being anyone of no specific identity.)

As always I gravitate towards the least biased sources, like PBS and also do my own research if it interests me. I also like to read news from other countries, which is a bit disturbing sometimes, like the Australian media vs Biden/Harris.
I’ve noticed I’m tuning out more, though, which isn’t good and I certainly hope my fellow American’s aren’t doing the same thing.

That is a great question. I have lost a lot of faith in most news media. There is one which I believe is the most truthful.

I compare notes with family and friends.

I use what I believe to be trustworthy sources with a reputation to consider and if I am told something that surprises me or that I find hard to believe I dig a bit deeper. It seems that truth isn’t valued as it once was. My generation would feel ashamed if caught out in a lie, now it is passed off with a smile. I think the rot started with advertising and has spread into political life and of course social media.

I check with several trusted sources.

The sort of mental heuristics I use to assess information really depends on the nature and context of the information.

If it’s some sort of scientific claim, then I’ll want to see if there’s peer-reviewed research, if it’s been replicated, if there’s meta-analysis. A single piece of research can easily be wrong, or superceded by newer studies, for example. At least in science, the more the scientists are sure of something, and the more of them agree, then it’s usually a good sign that it’s ‘true’.

When it comes to more general human affairs, society, politics, economics, etc, then things get muddy, because it’s nearly all value-laden. The motivations are more about upholding comforting worldviews and gratifying egos. ‘Truth’ in these realms may be viscerally opposed by the majority. You can’t trust the popularity of an opinion or even the number of sources, because they can reflect common doctrines and values. Here the dissident views can be more correct, or nearer to the truth, than the popular ones.

Sometimes I just stop and think about what it is I’m reading or seeing. A lot passes into people’s heads it because they don’t even consider that it might be untrustworthy.

I suspect being able to detect irony might be a related and relevant skill, because it also requires questioning and assessing the information, and being open to the idea that it’s simply a joke.

I don’t sweat it anymore. When some idiot walks up and starts sprouting BS I just come at them with my own BS. My wife and I took the two year old grand baby to the play park a few weeks ago, and some dumb ass got out of a car next to us, and asked why we were wearing masks. Started the shit about, well I think it’s all a plot to get us in line like sheeple, only liberals do that. I replied, I think Covid is a plot by conservatives to kill libs. You guys dont mask up because you have immunity. It was invented by Republicans in a secret government lab anyway. Lol – he looked at me like I was nuts and walked away. Good riddance and fuck him. Two can play the whackadoodle game…

Now I’m waiting for a chance to make my Big Reveal. I have it on good authority that Q of QANON is non other than Vladimir Putin himself. And don’t ask me to reveal my source. ; )