I think this is a gerenal issue: If you have a .bib-file with 200 entries and you use maybe about 170 of it, how can you retrieve the missing 30 keys?
The background is: \nocite{*} will add all references, but that's not what I want. I want only the references which are really used. But: I also want to know what keys are not used, so I can scan through that list and check if important references are missing.