You can also look at the first and last notes of the song. Typically, one or both of them will be the same note as the name of the key. So, for example, if the song ends on G, you might determine that the song is in G major and not E minor.
Sometimes, you'll find that the chords in a song will lead you to say the key of a song is a major key. But if the focus of the song is on a minor chord, then you'll need to use the relative minor key name. All these tips should get you started with finding keys on your guitar.
