I know this is more than a year since the author asked 2 be told how to know which key they're in, but I know this will help someone @ least..
firstly I dont think U will know which key U'r playing in until you know the scales..You neednt know them all, but just 2 know what they mean/ r..they define what key one is playing in (but i encourage you to know as many as U can..know all the major (if so, U already know all the relative minor. the relative minor scale is a matter of shifting the mode of the major scale..for instance, Gb (aka F#) major scale is Gb, Ab, Bb, B, Db,Eb, F, Gb..
the relative minor scale that applies the same notes is Eb...so the Eb minor scale is : Eb,F, Gb, Ab, Bb, B,Db, Eb..so 1 of the many ways of looking at each major scale is seeing it as a minor scale of notes 3 degrees behind it/ 1.5 half-steps behind..( hope my theory's correct..experts can correct..i only teach myself from the net).
without further ado, let me paste my attempt to answering the question as I've tried 2 explain to some ppl before:
[jrbrownz, thanx 4 allowing me 2 do this]
http://zone.hearandplay.com/modules/PNphpBB2/files/identifying_the_key_you_are_playing_in.docOk, this was during a chat session.
Hope it helps some1.