I like these sites for looking up when music is in the public domain:

http://www.unc.edu/~unclng/public-d.htm

http://www.copyright.cornell.edu/training/Hirtle_Public_Domain.htm

http://www.pdinfo.com/

There's also a mailing list that sometimes discusses the topic at

http://groups.yahoo.com/group/pdsongs

Quoting lines of songs not in the public domain is a grey area that may or may not fall under the categories of fair use or educational use.