This might not be the answer if you've never had any interval ear training, but it would be good to add it here.

What I usually try to look for is a V-I drop. In a lot of songs, it is used to determine the scale.

In some classical pieces, it used somewhere in the beginning of the song. For instance, if the song is in C natural minor, there would be a G - Cm drop. That would help you understand the scale.

It is quite often seen as: IV-V-I (and in jazz, II-V-I)

But, in some more complex songs, this might not exactly point out the scale.

This might not be the answer if you've never had any interval ear training, but it would be good to add it here.

What I usually try to look for is a V-I drop. In a lot of songs, it is used to determine the scale.

In some classical pieces, it used somewhere in the beginning of the song. For instance, if the song is in C harmonic minor, there would be a G - Cm drop. Whereas, if the song is in C major, there would be a G-C drop.That would help you understand the scale.

Εxample:

In C major and C harmonic minor scales, the V chord is the same. It's G-B-D. So, if I hear a G and then a C/Cm, I would know that the song is in C major or C harmonic minor scale respectively.

@AlexanderTroup, my teacher played a lot of V-I's on the piano, some where minor and some major. That was my ear training material. (There were more chords, not just V-I).

It is quite often seen as: IV-V-I (and in jazz, II-V-I)

But, in some more complex songs, this might not exactly point out the scale.