For news we can assume that it's signed by the journalists and news agencies who created the content. Or who researched the origin and are willing to put their credibility on the line.
Edit: To clarify, this doesn't prove if something is real or fake. It just makes attribution of the origin easier and creates a traceable chain of trust.
Maybe with a chain like with SSL certificates & authorities? Or maybe there could be a reference database with all the versions signed by a given authority so that you could verify it's an approved version? Why not go even further and show the same kind of alerts / block content if the source / signature can't be trusted?
Edit: To clarify, this doesn't prove if something is real or fake. It just makes attribution of the origin easier and creates a traceable chain of trust.