I tried by simply matching the tone of a note in a slower song and just went from there. You'd be surprised by how easy it actually is, AND how many bands really play the same notes, scales, and riffs. E minor is the stereotypical metal chord...everyone knows that. You can then expect various power chords that don't change very much at all...maybe a ghost chord or something like that...in case you aren't familiar, a ghost chord is simply playing a power chord but skipping a string and muting the skipped string with your left hand. A good song to practice ghost riffing is "Beyond Within" by Nevermore. Very straightforward and repetitive. If you aren't on the correct note, you'll definitely know it. If it's correct, it will resonate with your ear and will sound really fucking nice when you play along to your cds. I just bought "The Guitar Grimoire" book, and it explains a whole lot about theory, scales, chords, modes, etc. I recommend buying it.