all u need to look for is the root note of a riff...i do this by feeling thru the fretboard and play a little scale to see if it matches...and then i mess arnd with it for a bit...then istart exploring more by moving up the frets and probably end up using a different scale..i mostly use pentatonic...the example u gave woudl probably refer to G root note...im not really a theory person i cud be wrong i jsut do it thru feel really...hopefully the other guys would come help...hope this helps tho