SQL> select * from t where id=0 for update nowait;
select * from t where id=0 for update nowait
              *
ERROR at line 1:
ORA-00054: resource busy and acquire with NOWAIT specified

select upper(c.object_name), a.username us, a.osuser os, a.sid, a.serial# 
  from v$session a, v$locked_object b, dba_objects c 
 where /*upper(c.object_name) = upper('&tbl_name') 
   and */ b.object_id = c.object_id
   and a.sid = b.session_id;