Check the key signature of the music, between the clef and the time signature. The key signature consists of a series of flats and sharps that tell you how to play a given note when it appears in music. If you see a flat symbol on the B line, you will always play the note B as a flat.
You can determine the key of a song by looking at the number of sharps or flats in the key signature, according to Piano Lessons Info. For sharps, from one to seven sharps, the sequence runs as follows: G, D, A, E, B, F#, C#. For flats, it runs F, Bb, Eb, Ab, Db, Gb, Cb.
Count up a half step from the last sharp in the key signature, or count down five half steps from the last flat to determine the root note of the song. If you have no sharps or flats, the song plays in the key of C major.
To find the relative minor root, count three half steps down from the root note. For example, the key of C major corresponds with A minor.
To verify you have the right key, play the scale associated with the minor root over two octaves. In the process you'll produce the relative major scale. For example, the A minor scale over two octaves will encompass the C major scale.