Declare
   is_locked   boolean      := false;
   dummy       VARCHAR2 (2);
Begin
   Begin
      Select     null
            INTO dummy
            from mytable
           where rowid = :myblock.rowid
      for update nowait;
   exception
      when Others then
         If sqlcode = -00054 Then
            is_locked  := True;
         End If;
   end;

   IF is_locked then
      Showmessage ('Another user is updating this record. Try again later.');
      raise form_trigger_failure;
   Else
      LOCK_RECORD;

      If Not Form_success Then
         RAISE form_trigger_failure;
      End If;
   End if;
End;