The public domain is the term used to describe works whose copyright term has expired in the United States, which means that these works can be freely copied. When looking at a copyrighted work finding out whether it is in the public domain is often the first step in determining if you can use that work or if you need to seek permission.
This topic can be a little complicated, but a basic rule to remember is that copyrights have expired in the United States for any works registered on or before 1923. Another basic rule would be to obtain permission for any work published on or after January 1, 1978, unless you are certain the work is in the public domain. For more complex determinations, I have listed some dates and rules below. However, it may be helpful to refer to a chart such as this
For works published by United States authors on or before December 31, 1977, the maximum term of copyright protection is ninety-five years. Copyrights obtained from 1923–1963 have a ninety-five-year term if these copyrights were renewed, while copyrights obtained from 1964–1977 benefit from automatic renewal and definitely have a ninety-five year term. Creators should also keep in mind that foreign countries may confer different terms of copyright protection.
For works published on or after January 1, 1978, the term of protection is usually the life of the creator plus seventy years, so these works would only be in the public domain if copyright notice had been omitted or improper. The absence of a copyright notice on works published between January 1, 1978 and February 28, 1989 (when the United States joined the Berne Copyright Union) does not necessarily mean the work entered the public domain. On or after March 1, 1989, copyright notice is no longer required to preserve copyright protection, although such notice does confer some benefits under the copyright law.
For an hourly fee, the Copyright Office will search its records to aid in establishing the copyright status of a work. Copyright Office Circular R22, “How to Investigate the Copyright Status of a Work,” explains more fully what the Copyright Office can and cannot do. Circulars and forms can be ordered from the Copyright Office by calling (202) 707-9100.