On master of Puppets, Hetfield is just playing a riff in E minor. The entire solo is in E minor as well. So basically if you see the rhythm player playing a riff in E minor, then you know to solo using the E minor scale, melodic minor actually. Knowing the key of what someone else is playing in is easy if your practice your scales and know them on the fretboard. Knowing basic theory will help out too. Picking the right notes depends on what sounds good to you, but if a rhythm is in E minor, then you want to pay special attention to the notes in that scale when you solo.