✔ 最佳答案
The US Copyright Office has a database of registrations and renewals that might be useful. If something was published prior to 1963 (with the required copyright notice) then it also needed a "renewal" at 28 years or it became public domain. There are estimates that 70 percent of all such materials were NOT renewed, so you will not find any renewal registered in the database, making them (automatically) public domain.
In other words, if you find a film with, say "copyright 1951" and there is no corresponding 1979 renewal in the record, then (in theory) there is no more copyright on it.
There are other online databases that collect info on "public domain movies" for your convenience.