The last changed time comes with the assumption that the web server provides accurate information. Dynamically generated pages will likely return the time the page was viewed. However, static pages are expected to reflect actual file modification time.
This is propagated through the HTTP header Last-Modified. The Javascript trick by AZIRAR is clever and will display this value. Also, in Firefox going to Tools->Page Info will also display in the "Modified" field.
In general, there is no way to know when something on another site has been changed. If the site offers an RSS feed, you should try that. If the site does not offer an RSS feed (or if the RSS feed doesn't include the information you're looking for), then you have to scrape and compare.
01. Open the page for which you want to get the information.
02. Clear the address bar [where you type the address of the sites]:
and type or copy/paste from below:
javascript:alert(document.lastModified)
03. Press Enter or Go button.
Take a look at archive.org
You can find almost everything about the past of a website there.
Source: Stackoverflow.com