its too simple, you can just type "cache:" before the URL of the page. for example
if you want to check the last webcache of this page simply type on URL bar cache:http://stackoverflow.com/questions/4560400/how-can-i-get-the-google-cache-age-of-any-url-or-web-page
this will show you the last webcache of the page.see here:
But remember, the caching of a webpage will only show if the page is already indexed on search engine(Google). for this you need to check the meta robot tag of that page.