Yes, you can use HTTP_REFERER to do this. It's really the
only way, except for asking the users directly. And it is
very easily faked or blocked. Still, the majority of
users know not how to do this, so you can probably
grab a lot this way. Parsing your access logs would
be the best way to grab this, of course. Finally, make
sure that your server is actually *logging* this. It usually
appears as the last field in the access_log for apache
servers.