First, use this form (http://www.scils.rutgers.edu/~lesk/copyrenew.htmll).
This form searches the U. S. copyright renewal records database. Any book published during the years 1923-1963 which is found in this file is still under copyright, as are all books published after 1964 (although until 1989 they still had to have proper notice and registration).  Books published before 1923, or before Jan. 1, 1964 and not renewed (in the 28th year after publication), are out of copyright and therefore in the public domain. The form only searches books, not music, etc.

Cornell University has provided a very helpful chart to determine if an existing Work is still covered by copyright, see
http://www.copyright.cornell.edu/training/Hirtle_Public_Domain.htm