Wikipedia
Wikipedia has become a source of information for millions, but it is not without its problems. Vandalism occurs, and many authors think they know much more about a topic than they truly do. Many authors have political or commercial agendas that they build into Wikipedia articles. As most of the authors are anonymous or pseudonymous, it is difficult to evaluate the content of an individual article.
How Wikipedia Works
If you use Wikipedia, may I suggest that you refer to How Wikipedia Works. Chapter 4 is titled, Understanding and Evaluating an Article. This is the most useful chapter for professional researchers. If you haven’t used Wikipedia much, may I also suggest reading Chapter 3, entitled Finding Wikipedia’s Content, which is about searching.
Article History
Go to the top of the article and click on the tab marked history for a list of the people who have changed the article. You can compare the changes to see what changed in each version of the article by using the radio buttons. You will often notice that certain “contributors” are really vandals and knaves.
Look at the History and Discussion tabs for the article on the Danish Muhammad cartoon controversy to see how these can be really important. You can quickly tell who not to trust from reading the discussion pages and looking at the history tab content on many articles. For example, look at the contributor RanEagle in the history tab and click on talk.
Sockpuppetry
Mark Schaver writes on the Depth Reporting blog about a site, WikiScanner, that reveals organizations where employees have made anonymous edits of Wikipedia articles.