1. If the work was published in the United States prior to 1923, it is in the public domain.
2. For works published between 1923 and March 1, 1989, it depends on whether the certain statutory formalities were observed, such as providing a notice of copyright on the work or renewing the copyright per statutory deadlines. Examples:
down below for examples
If the work was published in the United States between 1923 and 1978 without a notice, it is in the public domain.
If the work was published in the United States between 1978 and March 1, 1989 without a notice and registration, it is in the public domain.
If the work was published in the United States between 1923 and 1963 with a notice, but copyright was not renewed, it is in the public domain