Turn debug logging on to either the console, the internal buffer or best to an external syslog server, it'll give you every URL that a user requests through the PIX. It'll also show you every connection and translation created so you can see where everyone is going. Basically you want something like:
logging on
logging trap debugging
logging host inside x.x.x.x
Then set up host x.x.x.x as a syslog server, you can get free syslog SW from www.kiwisyslog.com if you don't have any available.
Logging command reference is here:
http://www.cisco.com/univercd/cc/td/doc/product/iaabu/pix/pix_62/cmdref/gl.htm#1028090