So glad you asked! It's pretty much on a case-by-case basis, and there is no easy guidelines to tell whether or not infringement has taken place.
Fair use means that you get a legal OK to use copyright works under very specific conditions. The U.S. Copyright Act uses four key criteria to determine if a use is fair, or if it's copyright infringement:
To get a little deeper into the subject, head to the U.S. Copyright Fair Use Index, a government website that discusses various court decisions related to fair use principles. The website is also the source of all the information above.