Many sites use a “new” and “updated” icons or “posted at” and “updated at” marking when some content is added or edited at the site. But not all of them do that. So, how do you know if anything changed on the website since you visited it last time? Well, if you need it, I have an answer for you. www.WatchThatPage.com provides the service that you are looking for. They can send you the updates via email or you can see them on personalized webpage.
Now I have two news for you. The good one is that the service is free. The bad one is that I haven’t tried it myself, but someone recommended it to me. If it doesn’t work as advertised, you are on your own. If you tried it and say anything about it, please do so by all means (using the comments).