You want to play along with a song but you don’t know what key it’s in. How do you figure it out?
The simple fact is…most popular or traditional songs end on the root note of the major scale they are written in. This is known as the “anchor tone” around which the song is built. You can think of it as the “home” to which the song returns at the end of most phrases, and, most importantly, at the end of the song.
This is the “key” of the song.
In the example given here, “Amazing Grace”, notice how the song ends on the note “G”. This is the key of the song.
When you don’t have the notation, or an indication of what the chords are, figuring out what note this is can be a challenge. You just have to peck around on your guitar until you find the note. This requires at least two things: an ear for telling the difference between tones, and an understanding of the notes on the fretboard. Both of these things are pretty much necessary to play the guitar beyond the raw beginner stage.