Works published or registered before January 1, 1978, had a maximum of 95 years of copyright protection -- so anything published before 4/23/26 (as of today) should be public domain. The copyright owner had to renew their copyright to get that maximum benefit, though, so with research one could find plenty of things published after 1926 that crossed into the public domain.
The initial copyright term was 28 years, so anything that wasn't renewed became p.d. long ago.
https://www.copyright.gov/circs/circ01.pdf
Copyright law changed significantly around 1978.