#1
can anyone tell me or direct me to a lesson teaching how to know what key a song is played in??
#2
I'ts to do with the chord progression underneath you need to learn some theory to work this out for yourself and be able to do it in any situation.
#3
Learn key signatures and chord progressions. The first or last chord is more or less likely to be the tonic chord (chord built over the root note of the key) so try that out.