How can I determine what chords are in a key?

To determine the chords in a key, you can use the formula of building chords based on the scale degrees of the key. In a major key, the chords are typically built using the 1st, 4th, and 5th degrees of the scale as major chords, and the 2nd, 3rd, and 6th degrees as minor chords. This formula helps you identify the chords that naturally belong in a specific key.