http://guitaralliance.com/guitarmojo/blues-o-matic/key_to_the_highway/broonzy_big_bill-key_to_the_hig.pdf

Here's a post that somewhat goes into this topic-
http://www.harmonicalessons.com/members/dcforum/DCForumID4/37.html

The chord that the song "resolves" on is it's key. This is almost always the last chord of the song and many times the chord at the end of the chorus. Sometimes, it's also the very first chord of the song, but this is not as consistent as the others. Here's the definition of "resolution":

Resolution- Whenever you return to the "key note", that is, the note that is the same as the key of the song, this is called resolving or resolution. It typically tends to happen at the end of a verse or chorus and almost always at the end of the song. Basically, resolution lets the listener know that you've finished.

Essentially, time and practice and these tips will help you find the key of songs quickly. But, mostly practice.

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