Major and minor scales
Tell a major scale from a natural minor one by ear.
Two scales
Major and natural minor scales both have seven degrees, but their tones and semitones fall in different places. That is why one sounds bright and the other dusky, just like the triads built on them.
The C major scale
The C natural minor scale, from the same note
Practice: Which scale?
Listen for the third degree
The first three notes of a scale already give the mode away. In major, the first to the third degree is a major third; in minor, a minor third. Further on, minor darkens even more: its sixth and seventh degrees are lower than in major too.
The start of a major scale, then a minor one: C–D–E and C–D–E♭
You don't have to hear the scale to the end: try to answer after the third note, and use the rest as a check.