According to the intellectual property lawyer Ivan Hoffman: "if a work was published on or before December 31, 1922, then it is likely to be in the public domain. But you should never rely solely on this date alone since there may be other factors to consider."
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.

