Once the copyright on a work expires, the work is considered to be in the public domain and can be freely duplicated, used, and altered. The following charts explain when a work will enter the public domain.
This Copyright Genie http://www.librarycopyright.net/resources/genie/ allows you to determine if a work is protected by copyright.
The Fair Use Evaluator http://www.librarycopyright.net/resources/fairuse/ helps you better understand how to determine the "fairness" of a use under the U.S. Copyright Code.
"Exceptions for instructors" http://www.librarycopyright.net/resources/exemptions/ assists in identifying if an intended use meets the requirements of the copyright law.