Type some of the lyrics in any search engine and see what appears. The engine may lead you to one of the many song lyric websites or the discography of an artist. If you know the artist's name, type that in. You also can browse Napster and type in the artist's name. A list of songs the artist recorded will come up, and you might find the title on it.
Log on to song lyric websites such as Song Lyrics or Find Lyrics Free. You can type in the artist or a few of the words in the sites' search engines. Your search will pull up a list of songs from their database, usually with a sample.
Create a free iTunes account and search the store by artist or lyrics. A list of songs will populate that matches your search requirements with a sound bite of each song. You can click on the free sound bites and find the song.
Visit the local music store, particularly if it's a large chain, and ask a clerk to enter the artist's name in the inventory search engine. If the store carries the title, you might recognize it on the list.
Look on the website of the radio station where you heard the song if it was recent. Many popular music stations keep their playlists online. You will have to check it as soon as possible because they aren't posted online for a very long time.