When looking at Users in the Schema Browser, the Proxy tab shows what users can connect as this user. The SQL it's running is basically SELECT * FROM DBA_PROXIES WHERE CLIENT = 'THIS_USER'.
I think it would be useful to also be able to drive this the other way round, so I can see what users this user can connect as. I think the SQL would basically be SELECT * FROM DBA_PROXIES WHERE PROXY = 'THIS_USER'