You might be able to use Request.ServerVariables("HTTP_REFERRER"), but that would not work all the time (If they just type in the url rather than follow a link). Or you could check their IP address Request.ServerVariables("REMOTE_ADDR"), which is also not reliable as many users will have dynamically allocated IP addresses. So your best option would be to get them to log-in to your site. Then you would know exactly who they are.
Sorry, I'm very new at this, can you give me some more ides on how to use the Request.serverVariables("HTTP_REFERRER"). Can I use it in the document.open() method, so it would trigger every time the window is open?