Axis of Awesome - 4 Chords

This video is very funny but also gives a good insight into how to play chords by ear without even listening to the bass line. The progression they're playing is I - V - vi - IV, which they've transposed to E so it becomes E, B, C# minor, A. Now if you hear a song that has the same progression just work out what key it's in and - hey presto! - you know what the first 4 chords are.

Warning: contains strong language!