how determine last time given db changed in any way?

Hi,

   I was wondering (for the purposes of doing backups) how I can tell
the last time part of a db was changed, be it actual data being
inserted/updated/deleted or a table definition being changed, etc.  Is
there one timestamp in a system table somewhere, or maybe one timestamp
per table or do I have to examine the file system?

Thanks.

Joe Hudson
Chief Something-or-Other