•   The end of the URL (Uniform Resource Locator) will give you information -

•   A tilde ~ indicates someone's personal web page.

  Example: http://www-hsc.usc.edu/~bjmcmil/
   

•   The beginning of the URL may give you clues to the company or source through it's domain name.
  
  Examples: http://www.microsoft.com/
  Examples: http://www.barnesandnoble.com/

Then, take a good look at the author of the site:

•   Is information about the author or creator of the site published?  What are his or her credentials (education, training, job title, experience)? 

Example: http://antoine.frostburg.edu/chem/senese/101/redox/faq/lemon-battery.shtml (Click the Home button, then look at the bottom of the list along the left side of the screen...)
 

•   Is contact information given (author address or email)?  Look for "Contact Us"...

Example: http://hms.harvard.edu 

Now, use IMSA's 21st Century Information Fluency Project to view more ideas on this subject as well as quiz yourself.  Once at IMSA's MicroModules site, click on "Author: Who wrote this web page?"
 

Finally, take a look at some examples in the light of this information.