I get a lot of music from online sources, and most of the time they are missing things like intro's outro's and bridges. I find that listening to a song helps me an awful lot to figure out what's missing.
As it turns out, western music is pretty formulaic. Most songs use rotations and repetition in fairly a predictable manner. Usually, the song will have all the clues you need to figure out what's missing. Sure, there are exceptions to the formula, but they are so few that they might as well be non existent. Generally speaking if a song has a recognizable rotation all the little fiddley bits are going to be pretty close to the main structure of the piece.
I would recommend a using a
Circle of Fifths. This is a song writing tool, and a lot of songs are written using the chord structures outlined on the Circle of Fifths. The Cof5 simply shows how chords relate to each other. Each wedge contains a chord family of a Major, a 7th, and a minor. The Major and 7th are the same key, but the relative minor near the middle of the circle is a the next closest family of chords. So, say you have a song with a CFG rotation, and there is a mystery chord in the intro. If you systematically work through the relative chords you should be able to identify the right chord. For instance, if the mystery chord isn't a C7, F7, or G7, then you look at the minors. The minors are Dm, Am, and Em which are all commonly used with CFG rotations. But lets say you still don't have it. Since we now know from the Cof5 that the C, F, G rotation is related to the D, A, and E chords, you can begin to systematically work through those chord families, and so on.
Usually, that will work. But people like to get fancy with intros outros and bridges, so you may find yourself able to find the right chord family, but not quite nail the actual chord. Usually at a time like that I'll punt and use the closest chord I can find or change the arrangement to something that will make a substituted chord less noticeable. If the correct chord you're unable to find is actually something like an Add9, then you might still get away with using something like a C/F without anyone being the wiser...
