The URL of the page from which the request was sent is usually found in the "Referer" HTTP header in the
servlet request. So look at that header and compare it to what you want to see there.
Note that this isn't 100% reliable. It's possible to tell your browser to not send referrer information, and it's possible for a bot to send anything it likes as the referrer. However it's the best you're going to get unless you implement some kind of distributed single-sign-on system. And you did mention you didn't want to do a whole lot of work.
(And yes the name of the header is "Referer" and not "Referrer".)