How can I tell if tags have changed?

In my Write Exif module, I detect if the title or description has changed by listening for item_updated events and comparing $original and $new.

But, if I look at the tags there, I only see the old ones. Too early. The tags module triggers item_related_update, but apparently once for every tag, and with only the new list, not original and (final) new.

Is there something better I could be doing?