How do you tell what key a song is in?

1). From listening to the song, you have to be able to identify the "tonic", or the first note of the scale it's built on. It's hard to explain how to do that; best suggestion I can give is that it's often the note that the song ends on. 2). That note is the key that the song is in. If you don't have 'perfect pitch' (and most likely you don't because few people do), then find out what it is by matching it to a note on a keyboard or guitar.