The basic rules for evaluating a source for quality are the same as the "5 W's" of journalism: Who, What, When, Where, and Why.
Who produced the doucment, image, video, sound file, etc. and what do you know about them, their credentials, and their motives?
What is the factual or emotional content of the source and does it reflect reality?
When was it produced and does that time frame alter its potential usefulnes?
Where was it published and does that publisher evaluate sources befor publishing them? Does the place of publication reflect on the competence or impartiality of the source? Is this a re-publication and, if so, where was it originally published?
Why was the item produced and published? To educate? To influence? To sell something? To promote the creator?