Your method of sharping the last sharp of the key signature (to get the root note) is applicable to music in general and not only restricted to rock music.
One way to distinguish a major scale from a relative (C major - A minor) is to look out for a sharpen 7 note in a piece. If there is a sharpen 7 note , then most probably that piece is in a minor key.
For example :
C major - C , D , E , F , G , A ,B , C (octave)
A natural minor - A , B , C , D ,E , F , G , A
If the piece has a sharpened G in it (G#) , its probably in a minor scale considering the fact that G is the 7th note of the minor scale.
So why G# and not G? Cause there is another type of of minor scale called the harmonic minor scale which has a sharpened 7th note in it and that's why people often look out for a sharpened 7th note in a song to distinguish between the song is in a major or relative minor scale.
One thing to note is that , know your major scales inside out first. When you have identified the key signature of a song (assume it is in a major scale first) , you immediately check to see if there is any sharpened 7th notes. If there are , then its probably in a minor scale instead of a major scale.
