Ask yourself these questions:
What is the purpose and goal of the resource?
Who benefits from it?
Why was the source written or created?
Sometimes a book or video or website is created not to inform but to forward an agenda or point of view and to do that the information or facts within the resource may contain errors or even blatant lies. So knowing the motivation of the person or persons involved with the resource is important.
A prime example are web sites or books that deny the Holocaust. Some of the motivations might be anti-Semitism or perhaps German nationalism wanting to show the Nazi period in a more favorable light. If you are actually studying the phenomenon of Holocaust deniers these sites or books might be valid to use because it's part of the actual study. However, in research on the subject of genocide or the Holocaust you want factual information and informed analysis and you will not necessarily get that with these resources.
Watch for agendas versus a resource that is written to inform. The site should not make your mind up for you but cause you to think and evaluate the subject in general. If you find a one-sided argument make sure to inform yourself about other opinions and analyses.
Also consider who benefits, whether from a business stand point or anything else. If Company A manufactures tires and makes claims on their web site about the tires of Company B it would be advantageous to consider the source. By the same token, if a magazine publishes these same claims and Company A is also a major advertiser it would certainly be, at the very least, a conflict of interest to claim the study was true. The facts may very well be correct - but it could be harder to determine that accuracy because of the conflict. Better to look at studies and comparisons done by magazines that accept no advertising dollare (Consumers Reports is a good example), thus making their publicaion beyond reproach when it comes to informing consumers about different products. Just be aware that these situations do exist and that you might want to steer clear of them as a result.