This site gives you the information you need to decide if a song is in the public domain. It also gives a list of songs known to be copyright free. Be aware that although a song may be in the public domain, a specific arrangement might not, so it is best to check with the copyright owner if listed.
The home page of the United States Copyright Office. Copyright terms are explained and telephone help-line numbers are provided. Copyright forms can be printed from this site, filled out and mailed to the copyright office. Follow the directions provided at the web site