Hear it. Name it. No looking at a fretboard allowed.
Reading a chord chart tells you what to play. Ear training tells you why it works — and lets you figure out songs, transcribe melodies, and follow harmonic movement without ever touching a chart.
Start with Intervals, Easy pool. Get comfortable recognizing a 5th from a 4th, a major 3rd from a minor 3rd. Once that's reliable, open up the full 12 and add melodic (one-note-at-a-time) playback — that's closer to how you actually hear music.
Chord Quality builds on intervals. A major chord and minor chord differ by one note — the 3rd. Augmented and diminished stretch or shrink the 5th too. If you can hear that one-note difference, you can hear the difference between any two chord qualities.