Howdy guitarpeople.

I've been wondering how to determine what key a song is in? Is there some method of working it out?

First chord of the song? The chord the song revolves around?
I thought it was the first and the last note of the song? If those are the same notes, then that is the key.
But that's what I think :p
It is often the first chord in the song, but not always so working out the key from that is pretty haphazard.

It is more often the last chord in the song, again not always.

I find the best way is to see where the song resolves by looking for cadences and using your ear to hear which chord is 'home'.

Of course, I'm assuming you're not looking at a key signature.
My name is Andy
Quote by MudMartin
Only looking at music as math and theory, is like only looking at the love of your life as flesh and bone.

Swinging to the rhythm of the New World Order,
Counting bodies like sheep to the rhythm of the war drums
Pay attention to the chord progressions. If they go A, D, E the songs most likely in A major since that is a very common chord progression (I-IV-V). Cadences are also another thing to look for. V-I is a very common cadence, so if the chord progression goes back to A that is another indicator that the key is probably A major.
I think the simplest way is to play different chords at the end then see which one it ends best on.

Alternatively, looking at the seperate notes of each chord, putting them together, then looking at the sharps and flats should give you a major key they fit into (though it might not, like in Radiohead's Creep).

Try and end the song on the I chord of the key and if it sounds right then it's probably in that key. If it doesn't then try resolving on the vi chord. If this sounds right then it's in the relative minor of the major key.