How can you tell just by listening if a song has shifted from lets say A minor to C major ( relative key )? Does it have to have a sense of resolution on the tonic chord of the new key or something? I am having trouble trying to hear this and I need some help on what I can do.
I'm just learning theory now, but from what I know you can tell when listening to it because it will sound a lot happier if changed to C major from A minor. All of the lead notes will be the same, but the chords going on in the background will be different to complement the lead differently.
After some practice you should be able to hear key changes. As you can imagine, changing the key from C major to A minor involves both shifting the key from major to minor and shiftin the tonic from C to A. It can be deceptive sometimes as there are shared notes/chords between them, but with some ear training you should get it. A good example is Little Wing - Hendrix, which shifts from minor to the relative major each verse, and back to the relative minor for the start of the next.
And no, Guitar Hero will not help. Even on expert. Really.