>How do you know?

Two ways :

1) Staggeringly rare real-world events are reported frequently -- so a good number of them, by virtue of their rarity in real life, must be false. There are daily threats of wide-scale violence, but only a very small percentage of these threats turn into a real event.

2) The impossible is often reported. These comments can be thrown away at face value by most people -- besides maybe the most dedicated 'researchers' (Alien abduction, magic , general occultism woo).

Now, if you're asking 'How do you know which threats are false?', that's a much more difficult question, and i'd be surprised if there was a generalizable 'answer', aside from full-on mind-reading.

one pretty obvious fact would be the adoption of "chan-slang" by people who have turned entirely serious in their advocacy of those extremist views, with they themselves openly advocating that those were the channels on which they were radicalised. A similar dynamic is playing out on youtube, as a recent ex-member of this subculture attests:

https://www.nytimes.com/interactive/2019/06/08/technology/yo...