The most common type of modulation is to a closely related key. There are several methods we can use to identify modulations to closely related key.

For major keys, identify the subdominant and dominant chords, as well as their relative minor keys.

For example, in C major:

For a minor key, identify the subdominant and dominant chords, as well as their relative major keys. Then, find the relative major key of the main key.

A minor:

You can also look for degrees II to VI of a major key:

Or III to VII degree of minor keys:

Closely related keys are neighbors in the circle of fifths: