I was just thinking about how there is a pretty good way to discern the key of whatever song, and probably guess correctly. The cool thing is that this way doesn't really require knowledge of theory, scales, or even key signatures. Simply put: A good portion of songs start and end on the same exact chord. If the first and last chords were "G major," then you can bet pretty safe money that the key of the song is "G Major." Ditto for any minor chord too, like "B Minor." [Relative majors and relative minors make things tricky sometimes]
This nice cheat works a lot of the time. However, if you guess the wrong key of a song, keep this in mind: Music might be pretty boring if all songs always ended on the same chord they started with!