For me, songs always have a tonal center that makes the key pretty well-defined. Once you get used to hearing it in songs, it's not so hard. Once in a while, you get a "Sweet Home Alabama" with a tonal center that's more open to interpretation, but most of the time it's quite clear.
Well stated, and a perfect example of a song that gets debated more than it's worth debating.
(I vote G BTW)
