Can DLookup be used to compare to a control value.

I've thought myself into a corner and am probably missing something
blindingly obvious so I'd appreciate being pointed in the right
direction.

I have an Access database that has linked tables to 2 SQL servers.
The main back end, I have full control of but the second one provides
read only access to the full contact details of about 500 contacts.

My main form is based on a set of publications that are for sale. I
have a set of sub-forms on a tab control one of which is the record of
sales for each publication.

Out of the contacts, there are about 20 non-subscribers and as I
cannot write to the contacts datanase, I have created a separate table
with just an ID and Name field using a combobox based on the read only
table.

When a new sale record is completed, the customer field is a combo
based on the read only contacts table, but it can also accept entries
which are not in the the list. I've put a large red label on the sales
sub-form to warn when a customer is a non-subscriber and to do this I
need to determine one of three possibilities -

1. The customer exists in the read only contacts table and is not in
the non-subscribers table.
2. Diitto but is in the non-subscribers table
3. It's a customer who isn't in the contacts table.

In the second and third cases I want to display the label (which is
set not visible by default) and in the first I want to keep it hidden.

I started by trying to use DLookup to compare to "Me.Customer" in the
"after update" of the customer control but just got completely
befuddled.

Can anybody help please or point me to a working example of this sort
of problem.

Thanks in advance,
GA