Look at the key signature on the piano song's sheet music. The key signature is located right after the treble and bass clefs on the first staff of each line of music and explains how many flat or sharp notes, if any, the music contains using the symbols "#" or "b," respectively.
Determine which notes are played as flats or sharps. The flat or sharp symbols appear on the specific space(s) or line(s) of the note(s) to be played in a flat or sharp tone throughout the entire piece For example, if the B note is to be played in a flat tone, or Bb, a "b" appear on the middle line of the treble staff and the first line from the bottom of the bass staff.
Find the key of the piano song based on which note or notes are played as flats or sharps. For example, a piece of music that contains only one flat, say Bb, in its key signature is either written in the F major key or its relative minor key, D minor. Each major key has a relative minor key, both containing the same key signatures but different home chords, or I chords. The relative minor scale begins on the sixth note of the major scale, so the relative minor key of F major is D minor because D is the sixth note of the F major scale.
Determine whether the song is written in the major key or its relative minor key by looking at the first and last chords of the music, as one or both of these chords usually are the I chord of the key. For example, a song that contains only a Bb in its key signature is written in the F major key if the root, or first, position of the I chord is F. However, if the root position of the I chord is D, then the key of the song is D minor.
Double check that the song is written in the major or minor key by looking for any sharp, flat or natural tones throughout the piece that are contrary to the key signature. In the harmonic minor scale, the most commonly used of the three kinds of minor scales, the seventh note is raised one half step. When raised, the seventh note is played in a flat or sharp tone or in its natural, or regular, tone, depending on the note's original tone. If the music does not contain any flat, sharp or natural tones contrary to the key signature, then it is in the major key, but if the music does contain any flat, sharp or natural tones contrary to the key signature, then it is in the minor key.