Deleting inactive accounts

We have several home directory servers here and we’re trying to weed out inactive accounts. Unfortunately, as people have left the college their homes haven’t been deleted and their accounts are still active. Is there an easy way to determine the last time that someone has authenticated through Open Directory? If not, is there an easy way to see the last time that their home was modified? With either of these pieces of information we’ll be able to disable the other side and free up space on the server.