Depending on the year an item was published, the copyright renewal date might also be important. Stanford University Libraries maintains a database of Copyright Renewals from 1923-1963.
Government documents: works prepared by employees of the federal government as part of their jobs are not protected by copyright.
Is there a licensing option?
While licenses for use are often negotiated between the user and copyright owner, there are services that provide collective licenses such as the Copyright Clearance Center.
Creative Commons Licenses provide a way for creators to retain their rights to their works, while still allowing for free usage and adaptation of it. When a CC license is applied to a work, it means that anyone can use that work, as per the terms in the license, without having to contact the author/copyright holder for permission ahead of time.
Is there a specific exception in copyright law that applies?
US Copyright Law does allow for certain exceptions to the exclusive rights granted to copyright owners. The more relevant exceptions to instructional usage include:
Is this a Fair Use?
Another exception to consider is whether or not your planned use can be considered "fair." Fair Use allows for copyright-protected materials to be used without first obtaining permission from the copyright owner, or paying licensing fees, for the purposes of "criticism, comment, news reporting, teaching (including multiple copies for classroom use), scholarship, or research." Whether or not a use counts as "fair" is determined on a case by case basis by four factors:
The purpose and character of your use
The nature of the copyrighted work
The amount and substantiality of the portion taken, and
Who do I ask for permission?
If none of the above options apply, your last resort may be to contact the copyright owner. In order to do so, you'll need to figure out who owns the copyright (the author? the publisher?) and the best way to reach them. This may involve negotiating a license with the owner, which can include fees. They may also choose not to grant permission to use it or you may not get a response.