Learn the notes of the chromatic scale. You can start on any pitch when constructing a chromatic scale as long as you move in minor seconds. For instance, C to D-flat is a minor second and would constitute the first two notes of a chromatic scale.
Use the circle of fifths to match the number of sharps and flats in the music to the correct key signature. For instance, a minor piece that has three flats is in C minor. When using the circle of fifths, use the letters on the inside of the circle. The letters on the outside are for major keys.
Look at the first note of the minor scale and check the circle of fifths to see how many sharps or flats are in the key.
Decide if the scale has any notes outside of the key signature. If all the flats or sharps in the scale fit the notes in the key signature and the scale follows the pattern of major and minor seconds that are in the scale A, B, C, D, E, F and G, you have a natural minor scale.
Check to see any of the notes do not fit the natural minor scale. If you have a raised seventh scale degree, the scale is a harmonic minor scale. If there is a raised sixth and seventh on the way up, and natural minor descending, it is a melodic minor scale.