Key normalization

A few ideas:

• Use music theory to minimize accidentals (as proposed here)
  • This seems to work about 90% of the time
  • That's what this site does and this guy explains
• Use music theory to minimize accidentals to get candidates and then use music theory to choose intelligently from candidates
• Use neural network
• Use HMM

Interesting ear training and other techniques and sites:

• Minor chord test
• Forum about this exact question
• Lesson on key from chord progressions which uses the music theory to minimize accidentals
• Wikihow focuses on key signature, humming, and just practicing]

It's crazy how everyone essentially agrees there is a way to do it, but it always comes down to how it “feels”; is it “finished”? Where it “lands”. And other ambiguous phrases.