Finding out when a Web page has changed

Hi all, I need to write an application that accepts a list of URLs and checks them on a daily basis (via cron) to see if the pages have changed in the past day. I need some help with this. Does anyone know the most optimal way to find out when a particular Web page has been modified? I am thinking about using the Last-Modified: HTTP header - however, all servers do not return this header - any ideas on what the fallback should be? TIA, Vikram -- "I find your lack of faith disturbing." --Darth Vader