Google bot's last access for a website is very important for webmasters especially for people who have been having Google ads in their site. One of the easiest way to get Google bot's last access to a website is to sign up to googles webmasters tools .
That will be enough to provide sufficient data for your website.
If you wanna do it the rough way, then type site:websitename.com (without http://) in google web search. In the results which comes out , click cached link which is below every result . After opening this link you can see the google bot's last access to the particular website.