Robin is right -- there is no way to detect when a user bookmarks a page. If there were, we would easily be able to keep statistics on how many favorites were set -- but that just can't be done, unfortunately. It wold be useful information.
Thanks for the comments. Think I'm going to have to do a bit of coding in the pages.
Basically I'm trying to make sure visitors only bookmark the latest date dynamic page of some adverts where the url after the? determines the dates shown. Some of them are saving page 2,3 etc. by accident and then wondering why the adverts are a week out of date when they return.
The basic url is the same, with the stuff after the? changing to modify the dates.
So I was going to put a warning up if they try to bookmark page 2 onwards.
May have to code pages so that if they haven't arrived from our url, the page defaults to page 1.