No source of information is perfectly reliable. Some, however, are more reliable than others. Even when a secondary source is generally reliable, the careful researcher will always seek independent verification. (The following discussion applies mostly to secondary sources. For a discussion of the difference between primary and secondary sources, see the primary vs. secondary page on this site.)
Evaluating printed sources. When you pick up a book, consider the following:
Author. An academic historian, whose reputation depends on his being fair and accurate, may be likely to be more scrupulously accurate than a popular historian who is most interested in telling a story that will sell lots of books. Academic historians trained in the methods of historical research and practiced in the art of verification may also have an advantage over journalists who are trained to work with live sources but who often moonlight as historians.
Documentation. Is the book properly and thoroughly documented? Does it have footnotes, endnotes or some other system of telling you where the author got her information? Is there a bibliography? How extensive are the footnotes and bibliography?
Publisher. To be published by a university press, a book must be vetted by readers who are experts on the topic.
Date. More recent books build on previous works. The author of the latest work has the advantage of being able to incorporate and add to the findings of previous scholars. They sometimes also have the use of newly released or discovered primary sources.
Similar standards should be used to evaluate periodical sources. Articles in scholarly, footnoted journals (e.g. Journal of American History) should be given more weight than ones you find in popular magazines (e.g. The Atlantic).
Evaluating Internet sources. The Internet is a treasure trove of information—and MISinformation. Before using a website for a source, consider the following:
Has the site been recommended and/or vetted by a trusted source or publication?
Can you identify the individual or organization that produced the site? Sometimes the domain name offers a hint: in general, a .gov site is produced by a government agency; a .edu site by an educational institution, a .org site by a non-profit organization; a .com site by a commercial enterprise or private individual.
Can you discern the author's purpose? It's important to know if the author is an advocate for some cause or point of view. Does the author have an ax to grind?
What about Wikipedia? Wikipedia entries range from fraudulent to fabulous. When they are good, Wikipedia entries can have the best, most up-to-date, concise information available on the web. Occasionally hoaxers or saboteurs manage to insert false information or create false entries. A study by the respected science journal, Nature, comparing Wikipedia to the Encyclopedia Britannica in 2005 found only slightly more errors in the Wikipedia entries. To read about the study: http://www.wired.com/culture/lifestyle/news/2005/12/69844. Or check out the "Reliability of Wikipedia" article on Wikipedia itself: http://en.wikipedia.org/wiki/Reliability_of_Wikipedia#Librarian_views
Still, like any reference work, Wikipedia should be used as a starting point, not as your main source of information and should generally not appear in your footnotes. And because it is produced by anonymous volunteers, approach every entry with skepticism and seek verification.
Truth-telling websites. A few websites are dedicated to helping people separate the truth from rumors, lies, and bogus claims that populate the Internet and sometimes the mainstream media. I recommend the following sites.
Urbanlegends.about.com debunks modern myths, scams, and urban legends. Check out the list of top 25 hoaxes, the urban legend slide show, and the classic urban legends. See the definition of "urban legend": http://urbanlegends.about.com/od/errata/a/urban_legends.htm.
Fact checking sites that focus on contemporary politics include: the Annenberg Public Policy Center's http://www.factcheck.org/; and the St. Petersburg Times' http://www.politifact.com/truth-o-meter/.