> As someone who wishes to consume news that is as factual as reasonably possible,
There really isn't such a thing. What you report and how you report it is inherently editorial.
For example Fox News simply isn't reporting on the conservative protesters that are rioting in DC right now (stabbings, property destruction, etc), or choosing to report it as specific, isolated incidents stripped away from the larger context ("a man was stabbed today in DC"...). They aren't saying anything that's false, and yet their viewers largely have no idea what is going on in DC right now. And conversely they frame Seattle and Portland as war zones where you can't even drive down the street without getting a bullet, because they are reporting every single incident and every broken window there.
> how does one evaluate a news source for its trustworthiness, short of physically observing an event for yourself?
Well, live streams are a way that you can observe an event and the context surrounding it yourself, and observe the larger discussions happening on that in realtime (people talking about the stream on twitter/etc). But of course that's another filter bubble.
Also, virtually all official news media today has pro-corporate bias. Like, even MSNBC is owned and run by GE.
I would say it's more important to understand the biases of a particular source in general, and try to evaluate which is more credible in a particular instance.
But right now you can generally exclude most conservative-run media from any sort of credibility. Fox was outright established to be a voice for the Republican party itself (and there is no equivalent for Democrats), in the wake of the Nixon impeachment, because Murdoch felt that if the Republican party had been able to get its message directly to voters then Nixon would have been able to ride out impeachment (and he was right, this time it worked). From there the conservative media has gone off the deep end (OANN, Washington Times, etc) especially over the last 4 years. Criteria number one for credibility is "not conservative media", they simply have no interest in informing, only in defense and offense.