Is there a way to see, from within VIM, how a word was determined to be correctly or incorrectly spelled, for example, by displaying the spell file where it is listed?
I don't think there's anything built-in. You will have to write a function that fiddles with the 'spelllang' and 'spellfile' option values, and invoke spellbadword() to check whether it's still recognized. From that, you can deduce which source contains the definition.