Sue Medina of Network of Alabama Academic Libraries fame just sent this website which is a valiant attempt to clarify which publications are still under copyright and which are in the public domain. Public domain means that works can be used without permission, though they still need to be cited. If you happen to have the required info about the item, this is a nifty little web invention. Unfortunately most of the time one doesn't know whether the copyright was registered and extended or when the author died, and sometimes the answer given is just "maybe." You may have to do more research, but at least try it. Move the red slider next to your best guess and like the Magic 8 ball the answer may turn out to be correct.