Where does the book / article / website get its information?
Look for a bibliography and / or footnotes. In a piece of writing that is making a case using data, historical or scientific references, or appeals to outside sources of any kind, those sources should be thoroughly documented. The writer should give you enough information to go and find those sources yourself and double-check that the materials are used accurately and fairly by the author.
Popular news magazines, such as Time or Newsweek online, will generally not have formal bibliographies or footnotes with their articles. The writers of these articles will usually identify their sources within their texts, referring to studies, officials, or other texts. These types of articles, though not considered academic, may be acceptable for some undergraduate college-level research papers. Check with your instructor to make sure that these types of materials are allowed as sources in your paper.
Examine the sources used by the author. Is the author depending heavily on just one or two sources for his or her entire argument? That’s a red flag for you. Is the author relying heavily on anonymous sources? There’s another red flag. Are the sources outdated? Another red flag.
If references to outside materials are missing or scant, you should treat this piece of writing with skepticism. Consider finding an alternative source with better documentation.