To find the trace file one using oradebug one can find it using the example below:
SQL> oradebug mysetpid
Statement processed.
SQL> oradebug tracefile_name
/u01/oradata/admin/TEST/udump/test_ora_9964.trc
If you try to run the above command without attaching to a session it will return ORA-00074
SQL> oradebug tracefile_name
ORA-00074: no process has been specified