How accurate is the source?
Scholarly books and articles will include a list of works cited. Check to see where they got their information.
Non-scholarly sources will not have a Works Cited list, but it should be clear from the writing whether the ideas presented are the author's ideas or are taken from someplace else. For example, an article in a magazine about the current political climate in Iran may discuss the history of Iran. You can check encyclopedias to see if this information is accurate.