I'd agree not all of the Guardian's opinion pieces are trustworthy. This is true of a lot of news sources. I'd consider the ability to distinguish opinion piece

I'd agree not all of the Guardian's opinion pieces are trustworthy. This is true of a lot of news sources. I'd consider the ability to distinguish opinion pieces from factual reporting as one of the easy-to-learn stylistic differences to check, but I can see how that's unclear.