Actually it is extremely easy. It is easy to tell if an article has an angle or as not, by the choices of word and expressions. For instance, let's take the immigration crisis in Europe. Whether a media chose to call immigrants "illegal immigrants" or "asylum seekers" denoted bias in one direction or another. So left-wing media called them "asylum seekers" because they wanted to paint them in a good way, while right-wing media called them "illegal immigrants" to paint them in a negative way.
Now calling something "fake news" is biased too. All news are fabricated to an extent. "fake news" basically means , "news that don't come from an established source", it's new speak for unsanctioned news, to make it look like trusted news can only come from the NYT, Wapo and co ... it is extremely dishonest, since these media are propaganda organs.
> Opinion pieces don't count towards bias since they should be under a (labeled) opinion section.
Journalists are activists. Whether they write opinion pieces or not. The easiest way to tell facts from opinion is to take multiple sources opposed politically, and compare what they have in common and in which way they differ. Facts may be inferred that way. Off course it gets harder when the media choose not to report events because it contradicts the narrative they are pushing, and we all saw the result in the latest election. They can't blame "fake news" yet suppress information when it doesn't fit their narrative.
So obviously i'm not a journalist, but you could say I'm biased against MSM. More accurately I don't give them the benefit of the doubt just because they are professionals.