Peter - I'm not a music scholar, and can barely read notes, but here are a couple of ideas.

It sounds like you're mixing up CHORDS with the KEY. A song in the key of C, for instance, would generally have the chords of C, F, G (or G7) and sometimes the relative minor to C, which is Am. There are books which lay out which common chords go in each key.

A fairly good way to find the KEY of a piece of music is to go the last note of the song. A song or tune normally (but not always) ends on the tonic or "home" note; ie - C, for the key of C.

Or, if you've really astute, you can look at the beginning of the staff line and see how many sharps or flats (# or b) are indicated. This can also tell you the key. All I remember is that C has no sharps or flats. There are books that can clue you in to this system.

But, what you're asking is something I have often wondered too. How do you know where the melody notes indicate a chord change ? Most of us in folk music learned the "three chord trick" - once you know the key, your ear tells you which of the other two chords needs to be played. How, I don't know. You just hear it. Some songs, and some old time fiddle tunes can be done with just one or two chords.

Moving up in complexity, from the simple three chord songs, I suspect a music scholar will tell you that each note in any melody could have a variety of note clusters (a chord) that would sound more or less pleasing to the ear, and would harmonize and offer a "color" or "mood" - like all the minor chords and sevenths in Cockles & Mussels. That takes a little more ear training. Then you're approaching jazz.

What might be interesting for you to do is find a piece of music with simple chords and melody written out. Go through and determine which notes of the scale - a,b,c,d,e,f,g - the chord symbol is over. That might give you some insight and pattern ideas as to why the chord changes there.

I'll quit now - probably really muddied the waters.