First off is this just one database you are trying to clean up? How many objects are we talking and is it mission critical? Do you know what objects are definitely in use and where it is being used / by what? Really just start from what you know then she what is left, then maybe use object rights to effectively disable a few at the time and when you get a complaint you have a clue, if not mission critical. There are other ways to approach it but you might have to play dirty for a bit to get those answers.