There are three basic strategies that may help you in determining whether or not your use is fair:
Before considering whether you need to apply fair use or other limitations, be sure to determine if the work is still protected by copyright. A chart developed by Peter Hirtle at Cornell University explains copyright durations. While the chart outlines complex copyright duration cases, the general rule of thumb is that a work is protected within the life time of the author plus 70 years. Works published in the United States before 1923 are in the public domain and may be used without permission from the creator.
There are other limitations on exclusive rights that may apply in an educational environment. Section 110  lists exemptions for face-to-face teaching and distance education. Section 108 , which is currently undergoing study  and may be revised, and enumerates a few exemptions for libraries and archives. Consult the resources  section for more information about these exemptions, or visit the help  page for a directory of people at Northwestern who may be able to advise you. The Association of Research Libraries also has a "Know your Copy Rights " resource that provides a good overview of the questions to ask when using copyrighted works in teaching.