How about a 'subjectiveness' ranking instead, with notations for "unable to find link to a source of factual claims"? That's the best we could ever safely do, and we would likely find interesting results (WaPo might be labeled more subjective than Fox News, for example, even though both would generally have low numbers of notations for 'unable to find link to source of factual claim').
As I said in another post: The only thing you can possibly check for with any reliability is the validity of the base, source event, if there is one. Any higher derivation in the wrong hands (Facebook, for example) is systemic censorship. How can we safely do that? Recursive link tracking, perhaps, to a "primary source," which relies on the AI matching the factual claim to an appropriate source, either a video, image, or text that can be considered the legitimate source based on context (for example, an article about Apple's earnings could link to a primary source of their press release).
Regardless, the most dangerous form of "fake news" is by the selection/omission of stories, and that seems pretty impossible to quantify with today's technology (and of course the most prevalent on all of our 'real news' sources).