Re: Secret to find chords?
No ''secret.'' It takes a combination of a natural ear, and a learned understanding of melody, harmony, chord structure, and common patterns used in music (a.k.a. ''theory'').
For instance, on the bridges in that song, when you hear that chords are going IV to III and then II, you know that the III and II are minor, because you don't hear any accidentals (''out of key'' notes) in the chords. However, when the bridge wraps around to end on the V (which your ear quickly recognizes, as it is an extremely common way to end a section of a song), it passes through the II again. But this time you hear the telltale b5 accidental coming from the guitar and piano, telling you that the chord is major...and the tonic in the vocal harmony, telling you it's also a 7th chord. (The major third of a II chord is the flatted fifth of the key, and a tonic is the flatted 7th of the II.) This might sound weird until you realize that using a major II before a V in a major key is incredibly common, at least as far back as some baroque music I've heard, and probably even farther.
You wouldn't really consciously think it through like that forever, though. This is just a drawn out way of explaining the instant and almost unconscious listening/computing process that goes on in the brain of a practiced musician figuring out a song.
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