[AX7221]

A major scale is major, minor, minor, major, major, minor, dim or: I, ii, iii, IV, V, vi, viiº. You can use this in congunction with any major scale, take C major: C, D, E, F, G, A --> C, Dm, Em, F, G, Am, Bdimº7

You can do the same with the minor scale, it's pattern is: minor, dim, major, minor, minor, major, major, or: i, iiº, III, iv, v, VI, VII. So take the E minor scale: E, F#, G, A, B, C, D --> Em, F#dimº7, G, Am, Bm, C, D.

So once you know the patterns you need to get a table of keys from a website somewhere then do some cross referencing. Like fretscource said in another post this will tell you what groups of notes you're using, but your ear will tell you what the root is. For eg: E minor and G major contain all the same notes, so consequently all the same relative chords, so on paper you would have to make a guess to distinguish between the two.

What helped me remember the pattern is for major the 4 and 5 are major (same as the root), and for minor the 4 and 5 are minor (also same as the root). The dim chord is the 2 chord in minor progressions, and the dim chord is the 7 in major progressions. Then the rest are minor in the major key and the rest are major for the minor key.

However this is one hitch in the process, most songs have chords that are out of key. This gave me difficulties with house of the rising sun since all of the chords were in the key of A minor, except two of the chords Em and Dm were majorized into E and D. In short all these different things are great tools, but at the end of the day you gotta use your cunning, and creativity and style too.

One more thing, just say you have add9 chords, or dom7 chords and so on, at that point you can look up chord construction and write out the components for each chord, then see what scale you have. I've seen websites and stuff that do that for you, but I'm finding the more I work this stuff out myself the better my understanding becomes.