There is no way to find out which roles are enabled in a session, other than your own (which you can see in SESSION_ROLES). The reason is that this information is kept in each session's UGA, and for dedicated connections the UGA resides in the PGA which is in private memory, not visible to the rest of the instance.

I want to find out what roles are currently enabled for users logged on. The users are logging into Oracle from an application using a Web front end which issues SET ROLE for them.

I know about DEFAULT_ROLE in DBA_ROLE_PRIVS which shows what roles are set by default at logon. Also about DBMS_SESSION.IS_ROLE_ENABLED which is only useful for your own session. Neither of these give me the information I am after. Any ideas?