Using “oradebug dump events 1” one can find/list events set in a session.
For example:
In a session events 10046 and 1410 events are set
SQL> ALTER SESSION SET EVENTS='10046 trace name context forever, level 12';
Session altered.
SQL> ALTER SESSION SET EVENTS='1410 trace name context forever, level 12';
Session altered.
SQL> SELECT distinct sid FROM v$mystat
SID
———-
131
In another session login as an account as sysdba
SQL> SELECT pid, spid FROM v$process where addr IN (SELECT paddr FROM V$session where sid = 131);
PID SPID
———- ——-
12 3420
SQL> oradebug setorapid 12
Unix process pid: 10932, image: oracle@localhost.localdomain (TNS V1-V3)
SQL> oradebug dump events 1
Statement processed.
SQL> oradebug tracefile_name
/u01/oradata/admin/TEST/udump/test_ora_10932.trc
Or in 11g:
SQL> oradebug eventdump session
1410 trace name context forever, level 12
sql_trace level=12
After running the above oradebug will dump the events in the trace file in the above case /u01/oradata/admin/TEST/udump/test_ora_10932.trc shows event 1410 and 10046 events set.
…
Dump event group for level SESSION
TC Addr Evt#(b10) Action TR Addr Arm Life
974FED50 1410 1 974fede0 0 0
TR Name TR level TR address TR arm TR life TR type
CONTEXT 12 0 -1 2 0
974FEBF8 10046 1 974fec88 0 0
TR Name TR level TR address TR arm TR life TR type
CONTEXT 12 0 -1 2 0
*** 2010-07-15 23:40:05.671
..
An Oracle Spin by Alex Lima 
this is easier method from Julian Dyke
Listing Enabled Events
To check which events are enabled in the current session
SET SERVEROUTPUT ON
DECLARE
l_level NUMBER;
BEGIN
FOR l_event IN 10000..10999
LOOP
dbms_system.read_ev (l_event,l_level);
IF l_level > 0 THEN
dbms_output.put_line (‘Event ‘||TO_CHAR (l_event)||
‘ is set at level ‘||TO_CHAR (l_level));
END IF;
END LOOP;
END;
/