Sometimes, if you can find the chord that is played as a seventh (I.e., major or minor with a flatted 7th note in it), it is probably the dominant seventh. That means it is the 5th chord of the key. Once you know what the 5th Key chord is, just count up 5 HALF TONES (ie., 5 fretts) and the note that you come to (whatever it is) will be the ROOT NOTE or the main note of the key. In Major scales, the chords run M, m, m, M, M(usually played as a seventh chord), m, diminished, and finally Major. (M = Major, m= minor). E.g., the basic chords of the key of C major are: C Major, D minor, E minor, F Major, G Major (7th, or better known as the DOMINANT seventh), A minor, B diminished, and finally C Major again. This is the FORMULA for ALL major scales.
Check it out! :-))