How do I know which key a certain song is in? It's a common question from those learning the guitar.
Finding the key of a song isn't hard, it depends how much effort you want to make.
Here are some quick and dirty methods (which will work about 70% of the time). And a more academic way which works 99% of the time (there are always exceptions.)
QUICK AND DIRTY #1
Listen to the song and try to identify the first chord in the progression. This is often (but not always) the key of the song.
QUICK AND DIRTY #2
Listen closely to the progression and try to "feel out" on which chord in the progression feels "resolved". To identify this chord, imagine it as the last chord played when the song ends, being left to ring out and naturally fade. You might need to grab your guitar to test which chord feels most resolved.
THE CLASSROOM WAY (with cheat sheet)
Use this cheatsheet and plug in the chords from the progression in this matrix. You might not know all of them, but that's okay. If you've got 3 then this will work.
The chords in your progression will only fit one way in this matrix and the key will be revealed. So grab the cheatsheet now!