I don't see what's so difficult about finding the key to a song. When most songs end on the Key. Most people play by ear, so use your ears to find the key. But wait a minute, that's just the beginning because, the truth of the matter is a song may migrate through several keys. So how do you deal with that? Well the way I deal with it is I whip out the old tool box. Yeah, das right. A carpenter can have a tool box, so why can't I. LOL. If you look at written sheet music, you will notice patterns in the music. You will see patterns in the bass line for instance. It could be a 1,4,5 pattern. Or a 2,5, 1 pattern, or a 1,3,5,6,8. Also you will see patterns in the chords like 1,4,5 or 2,5,1 or chords like 7,3,6,2,5,1 which is really just a collection of 2,5, chords. These patterns are tools, building blocks. They are what songs are made of. Learn them in all keys cause, you will see them again. If you can't read, then learn what they sound like. Because you will hear them over and over again. I learn the bass pattern. I learn the chord pattern, through a melody over the top and walla, dare it is. So the best way, to learn a song is to break it down into patterns. I practice these patterns 'til I know them like the back of my hand.