You will first need access a properly tuned instrument that follows the Western music scale (usually a keyboard or guitar), and some good speakers. Rapid Evolution includes a built in MIDI keyboard which does the job quite well, and is useful for beginners because it displays the note/chord you are playing. You don't to be a skilled keyboard player to do this.
For beginners, it does not come naturally and it might be difficult to determine keys at first, but with sufficient practice and determination it becomes easy (like beat matching).
There are several strategies for finding a song's key:
This is the recommended method for keying songs. What you are looking to do is find the single note that most fits with the song.
Play one note at a time (you might want to experiment in different octave ranges). You will eventually find one note on the keyboard that fits better than any other regardless of what the actual song does. The note that blends in the most, practically disappearing into the song, is most likely the root note of the key, also known as the tonic note.
When you suspect a root note, you can play the subdominant and dominant notes as well. They should also sound good with the song.
Once you are certain of the root note, to determine whether it's a major or minor key play a major and minor chord based off the tonic note and see which sounds better. If necessary, you can play the notes of the major and minor scales based off the suspected tonic to be absolutely certain.
Here are some sample audio clips which feature the root note of the key being played on the piano with the song:
Play major or minor chords until you find the chord that blends most with the song. The subdominant and dominant chords of the same type should also sound good. It helps to stick to either major or minor chords, then to test whether the key is major or minor last. For example, find the best minor chord where its subdominant and dominant minor chords also sound good. Then, to determine if it's really minor or its relative major, play the tonic notes of each key and compare. If you are still uncertain, you can compare chords.
Here are some sample audio clips which feature the main chord of the key being played on the piano with the song:
A time consuming approach is to play notes of various keys (scales) until you find the key for which the least notes clash with the song. Then, to determine if the key is major or minor, compare the tonic notes or chords of each type. Remember that there are different types of major/minor scales, and so finding a scale where every note sounds good isn't necessary. The most important scale notes which determine the key are the tonic, the 3rd, and the 5th, i.e. the same notes that comprise the triad major/minor chords.
Here are some sample audio clips which feature the scales of the key being played on the piano with the song:
Once you have found the key, the tonic, subdominant and dominant notes should all sound good with the song. The tonic note in particular will blend in the most, almost disappearing into the sound picture, and your ear will eventually learn to recognize it. It sounds “at home” or centered and tends to resonate with the tones of the song. Finally, you should be able to play along with notes in the key and sound as if the original artist had put them there…
Most dance music (especially trance) tends to be in a minor key. The reason being that it is generally believed that minor keys convey more emotion.
Some electronica tracks are so melodically challenged that they could be considered minor or major with equal validity. Such tracks usually have just one or two identifiable tones, but no real melody. They have “no third,” which is essential is distinguishing between a major and a minor.