How to Identify users using a shared global schema in Enterprise User Security

So we are setting up Oracle Enterprise User Security using a global schema. One question that arises if I have ten users that  are using the global schema to sign on and run queries in a database.  If lets say ten users are running ten queries and one is bad. How do I tell which user it is? If the global schema is READONLY. When looking at the v$session table they will show the user as READONLY. How can I find out it is testuser1 or testuser2?