Hello Ron,
If you are setup for Warning and EUN, then you should be able to use the access logs search for 403 messages for those users (if auth configured) or their ip address.
The complete line of the log will also include the site visited and the category it matches.
example
1297321053.363 24 10.1.1.1 TCP_DENIED/403 1896 GET http://www.facebook.com/ - NONE/- - BLOCK_ADMIN-ericpc-ericpc-NONE-NONE-NONE - 0 0
This is what i get when setup for warning to go to social netowkring. Notice the TCP_DENIED/403, which is waiting for user acceptance of the warning. then the category snet for social networking.
I hope this information helps you.
Regards,
Eric