Here are links to the sites listed by Freightdawg. They seem to be the two most useful.
pdinfo
This site lists 3500 or so songs from 1922 or before that supposedly are in the public domain. Recent copyright statute revisions by Congressional Committee have changed the ground rules and pdinfo hasn't caught up. Put copyright in Lyrics and Knowledge search and look at previous threads. A real quagmire now.
By all means check out the revisions; go to Copyright Law of the United States

SongFile is one of the best if not the best on the web, but it won't answer all of your questions.

Posting titles here, because of the great variety of experience of contributors, may help you considerably.