Determining whether a work is still protected by copyright can be complicated. In general, whether a work is protected by copyright depends on when it was created or published. For most works created after 1978, copyright term, or the length of time a work is protected by copyright, is the life of the author plus 70 years, although there are exceptions for pseudonymous and anonymous works and works made for hire. 

The U.S. Copyright Office allows members of the public to search its copyright registration records. Keep in mind that these records only include those that have been registered, and registration is not required for a work to be protected by copyright.

Cornell University Library also has a very handy chart that can help you figure out whether a work is still protected by copyright based on when it was published or created.