Before you use a source, you should check to see if it seems credible and relevant to your topic. Use the title, abstract, publication and author information to see if a source looks promising, then read the text carefully, considering whether it is appropriate for your paper.

For instance, when searching for sources for an academic paper, you're usually looking for:

• Works by scholars or experts (authorities)-- for instance, for a literature paper, writers who are have advanced degrees in literature. 
• Works that have been carefully vetted, for instance, in a peer reviewed journal (you can limit your search to 'peer reviewed/scholarly journals')
  • The type of publication the source appears in matters. Newspapers and Magazines are less likely to be accepted; scholarly books, however, are often acceptable. Reference books such as encyclopedias and dictionaries are generally used only to verify facts or background information. 
• Works that are current with the scholarship in the field -- are they up to date, and factually, do they line up with other scholarly works on the topic? (As Daniel Patrick Moynihan said, 'Everyone's entitled to their own opinion, but not their own facts.')
• Works that cite the sources where they got their own information: if there's no bibliography or references, view it with extreme suspicion.
• Works that do analysis rather than expressing an opinion, and which are objective in their approach. ("Comparing the depiction of Hamlet in the Gesta Danorum and in Rosencrantz and Guildenstern are Dead" would likely be an analysis; "Tom Stoppard improves on Shakespeare's Hamlet" is more likely to be an opinion.) Some book reviews may be scholarly, but even scholarly ones are also to be used sparingly and with care. 
• And most importantly, works which are relevant to the topic you're working on. Even if it has all the right keywords and is scholarly, if the book or article has nothing to do with your topic, it's not going to help you. If it's an article in the Journal of the American Medical Association or Chemical and Engineering News on Ophelia, it's likely off-topic.
• There are other factors to consider as well. 

It's up to you to decide whether a source is useful; but your professor and the librarians can help you. Don't hesitate to ask!

Some Questions to ask:

• Where did this information come from?
• Is the author an expert in the subject?
• Does the author/publication cite its sources?
• Does the author or the publisher have an 'axe to grind,' i.e. a bias?
• Is the source (author and/or publication) positively regarded? 
• Is the information out of date?
• Is this the information I need?

More suggestions:
"Evaluating Sources" from Purdue's Online Writing Lab