getting the hash code and such shouldnt work, because every time you view the page there should be a small change in the HTML stuff (because of advertisements)....
well I would say that the upates on a site will most-likely be text only. They add/remove a link or two or add a bunch of text to the page. Images are usually advertisement, and I would say the CONTENT of a page hardly gets updated with images alone....
so a simple suggestion would be to read the html file, ignore all the image and other html tags (table, etc) and try to measure the length (in characters) of the visible text that appears on the html page. If there is an update to the page (which will again most probably be an update to the visible text on the page), then the length of that text would change....
if you dont know what I'm saying ask me to explain more it seems like a simple solution that should work. anything more complicated than that which would actually examine the page word by word is out of the scope of my IQ