The purpose of the author in presenting ideas, opinions, or research may in part determine the usefulness of the source. Does the source show political, cultural or other bias? Are opposing points of view represented? Is this information verified in other sources? You may not be able to evaluate the objectivity of any single resource until you have looked at all your resources. Even biased sources can sometimes be used, if you are aware of the bias. Find out more in Media Bias.
The book jacket or back of book may have information that can help you determine bias; articles may have information at the beginning or end of the article. The credentials of the author may give you clues to bias.
On Websites there may be an “about us,” or “about this site,” or “who we are” page that details what causes or ideas the site stands for. The Cato Institute states very clearly what their special interests are: