I look it up on youtube I type in the lyrics they said multiple times because a lot of times that is the title and listen to the song to see if that is the song. 99.9% of the time I find the song, the only time I don't really is if I forget the lyrics.
By trying to guess the song title from the song. (It's usually the phrase that's most repeated in the song, I've noticed...) If all hope is lost, google part of the lyrics.
I google the small bits and pieces I can remember of it and that usually works fine as long as you end the search with "lyrics".
Recently I got an iPhone though and I've been using the song recognition-thingie Shazam which is incredibly useful.