Go to a karaoke bar and sing a line from the song. You never know. A lot of these people are music buffs who might come up with the answer.
Type a line from the song into an Internet search engine. Place quotes on both ends of the line, and hit search. As an example, type, "Oh Say Can You See.'' The search engine's result is "The Star-Spangled Banner."
Go to an artist's (or band's) Web site, if you know who recorded the song. Many Web sites for bands list all their songs along with the words for each song. Scroll through the songs until you find the line.
Go to one of the multitude of Web sites that are specifically designed to help you find songs. These sites can find a name of a song by the words in the line. They also can often find the name by you typing in just a few keywords. There are even sites that can identify songs by you humming (or singing) the tune into a microphone on a computer.