combo box (not in list)

I have a combo box that the data comes from a table. I have it set so that
the users can enter information even though it isn't on the list, however I
want to know if the item is not on the list so that I can enable another
control on the form. It seems that when you allow items not on the list,
then the "not in list" event doesn't fire. Is this true, or do I have
something else set incorrectly? Is there an easy way to determine a "not in
list" event when you allow items that aren't on the list?