Using a PHP script tied to a mySQL database 
(Which stores name, site address, last checked, 
last updated, site hash, and catagory (EU, US, 
JP)).  The script goes through the database row 
by row, and checks the stored hash against the 
new hash.  If they're the same, then don't 
update last checked, if they're different, 
update last checked.  I'm using a modified 
version of wwwupdate