Whether a key is one that is actually written with a key signature.
Each of the seven diatonic modes has a signature of its own, and the harmonic
and melodic minor are the minor key with a degree or two raised: G# harmonic
minor is written with G# minor's five sharps and an F## in front of the note,
which is why it is spelled G# and not Ab. Every other scale only borrows the
signature of its parallel major or minor as an approximation, so nothing
about how it is written follows from that signature — and how such a scale is
spelled has to be decided from the scale itself.
Whether a key is one that is actually written with a key signature.
Each of the seven diatonic modes has a signature of its own, and the harmonic and melodic minor are the minor key with a degree or two raised: G# harmonic minor is written with G# minor's five sharps and an F## in front of the note, which is why it is spelled G# and not Ab. Every other scale only borrows the signature of its parallel major or minor as an approximation, so nothing about how it is written follows from that signature — and how such a scale is spelled has to be decided from the scale itself.