Go to pdinfo.com (Public Domain Information Project).
Click on the "Public Domain Music" link.
Click on a letter link to take you to a list of public domain music in alphabetical order, or use the search bar to search for specific titles.
Look for a date of publication for the music next to the song title. Some arrangements of songs are still protected under copyright, so you must make sure the specific arrangement you are looking for is no longer protected.
If you can't find the public domain music you are looking for online or can't find the publication date, visit the James Madison Memorial Building of the Library of Congress and search through the card catalog.
Go to copyright.gov/forms/search_estimate.html, submit a $165 fee, and fill out all applicable forms to find out whether a piece of music is protected under copyright.