The US Copyright Office will, for a fee, do a search to see if something is copyrighted.
Don't make assumptions based on quick calculations from the time of someone's death; copyright may have been extended (the classic example here is the Peter Pan book).
Go to Full Artist's Copyright FAQ.
Disclaimer: The information given here is based on US copyright law and is given for guidance only; you're advised to consult a copyright lawyer on copyright issues.

