pl/sql: test if record is empty

oracle 10G

I apparently overlooked it in the manuals - so please be patient ...

How can I test if a record is empty?

I expected it to work like this:

declare
  o_rec mytab%ROWTYPE;
begin
  if o_rec.empty then
    dbms_output.put_line('empty');
  end if;
end;

.... but it does not.

Thanks,
stephan