Media authentication is the tracking down of the source in media reports. The source may be quoted by media outlets in mainstream or local media or in social media space.
The evaluation of the reliability of the media outlet can tell you something about the source. For instance, a media outlet that is supported by the government will use a source that has a favorable bias towards the government, while an independent media outlet will most likely relay information from independent sources.
Search engines such as Google are useful tools to determine whether the source has been used by traditional media outlets or social media.