Q: How do I determine the key of a song?
A: This is not as difficult as it seems. You just take out your harps and play some notes and listen which one of them is the root note of the song. If you are quite sure that the song is played in second position (most blues is in fact played in second position) then you just try a few different keys of your diatonic harps and play the 2 draw. You will notice when you have the right harp -- BB
To help you in unusual cases, it might also be advantageous to obtain some music theory background, in addition to ear training in intervals (relative pitch), to familiarize yourself with common chords, progressions, cadences, modes, scales and other musical concepts. Some people have developed perfect pitch which is very helpful in determining keys first and relationships second. Most songs either start or end with the chord associated with the key center, that is, they either start out tension-free or resolve to tension-free (not always). Also a given song can be characterized as major or minor or modal, which of course can shift, but once knowing that, the tonal center or key can usually be identified by ear if you don't become confused by playing tones which support a main chord, but which are not the root note, or to play tones which are in different chords or sales that fit the progression.
Recordings that vary or tape decks that create slightly off-pitch conditions compound the problem. Variable speed tape decks help (Marantz, etc.) -- HA
(FMI: "Re: diatonics and song keys" 15 Sep 94 BL) (FMI: "Determining Key of recording" 25, 26 May 94 JE, WY, KG) (FMI: "Perfect pitch" 1 Mar 95 WY)