Watch the Find the Key! online guitar lesson by TrueFire from Play Acoustic Guitar 8: Advanced Techniques
Find the Key! - Harmony Fundamentals: 1 is a video guitar lesson presented by Massimo Varini and is sourced from Pop Guitar Survival Guide.
Before working with awareness on extensions of the chords we need to know the key of the song we're working on. Use the PDF attached to the lesson (called Work Sheet and Key Chart) and follow the instructions of the video. Watch the chords, spell out the chords on the Work Sheet, after writing all the triads notes observe if there are any accidentals (sharp or flat), go to the Key Chart to find the key that contains the changes you have found, write down the notes of the scale (key) at the bottom of the Work Sheet!