In addition to what Juan A. Navarro suggested, I'd say that words which occur in chapter and section titles are likely candidates for indexing. E.g., if section 2.3 is entitled "The Virasoro Algebra", then that's probably a sufficiently important topic that other occurrences of "Virasoro algebra" should be indexed. You could write a script (in your favourite scripting language) to pull out the arguments to \section commands and the like, throw out the prepositions and articles and sort the remainder by frequency. How your script will know that the words Virasoro and algebra go together . . . well, either you call import skynet and live with the consequences, or you do some manual work with its output.
Other things to check could include words which are capitalized when not at the beginning of a sentence and words set in emphatic type.