Having a list of addons makes upgrading much easier because you have a frame of reference for which files you would "expect" to be different from the original files. And which ones are intentionally "extra" files.
If you don't have the luxury of a list of added addons, you'll have to build your own by trial and error as you work your way through comparing your current site files against the original untouched files for the version you're using.
There's no hard-and-fast rule for identifying what addons exist in a 1.x.x site. Sometimes you'll get lucky and see readme files. Sometimes you'll see added files (not in original file set) which have filenames indicating what they're for. Sometimes the added files will contain comments indicating what the files are for which may give a clue to what addon they're part of. Sometimes altered files will contain comments next to any changes made which might give a clue to their origin.
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