True, a minor scale has to have a minor 3rd, being 3 half steps (3 frets) up from the root.

They also USUALLY have a minor 7th, which is one whole step (2 frets) down from the root.


And another thing that come into play is the fifth. If it has a perfect fifth, which is 7 half steps (7 frets) from the root, AND a minor third, it is minor.

Sometimes its good to just learn your basic minor keys: Aeolin, Dorian, Phrygian. In MOST minor keyed music you never see anything besides those scales/keys/whatever.

But you can usually tell by a third, but also check your fifth to make sure your not playing some weird diminished or half-diminished stuff.