Actually, it's not quite so simple a request as you may think.... The law has undergone several changes over the last century and different time schemes come into play depending on when the work was first published -- or whether it was ever published at all.
So first you'll need to find out if the work was ever published and, if so, when. Then, depending on when that is, you may need to determine whether it appeared with a copyright notice or whether it was ever registered with the copyright office--and, again depending on the time frame, whether that registration was ever renewed.
The Copyright Office has a circular on the duration of copyright at
www.copyright.gov/circs/circ15a.pdf .
But Cornell has laid it all out in a chart form which may be a bit easier to grasp at a glance. It's at:
www.copyright.cornell.edu/resources/publicdomain.cfm .
And note that with a song you may be dealing with multiple copyrights: for the music itself, for any lyrics, and -- if you're interested in a recording -- also for the particular recorded rendition of the song.
Good luck!
