Wondering whether your use is a fair use? While there no substitute for a legal opinion and no absolute black and white answers in fair use, you can certainly perform a relevant balancing test using this handy checklist from Indiana University.
Just go through the list and check the information that best matches your use. When you are done, print out the checklist and save it as a reference if any questions come up about your use in the future.
Also take a moment to read their useful Introduction to Fair Use, linked at the top of the page.