Fake news is generally identified by the reputation of its author (New York Times vs Alt-Right Patriot Freedom Observer). Nothing published by the Times or Wall Street Journal (despite any editorial bias) is fake, simply because we know (except in the rare circumstance) that they apply journalistic integrity in sourcing and writing articles. Much of what's published on "fringe" sites, especially if no mainstream outlets cover it, is likely fake (i.e. if it's making a factual claim about the world, not just expressing an opinion).
Obviously followers of these sites will fight back against any "fake" labels, by arguing there is a "cover-up" by organized opposition, that their truth is being suppressed, that any fake news classifier intrinsically has a liberal bias, and so on. The difficulty for me is not identifying what's fake, but convincing anyone else that it is.