Check using hints ot not

Hi,

I used this trigger to disable all hints in my application queries.

How can I check that all hints are ignored and that trigger works well?

create or replace trigger ignore_hints_on_logon
after logon on database
begin
execute immediate 'alter session set "_optimizer_ignore_hints"= true';
end;
/

NB: Trigger is INVALID at the end.  WHY?

Best regards.

GD.