Query For Duplicate Records

I am creating an ordering system which comprises of amongst others, these fields:

ID
OrderHdr
OrderLine

I want to ensure that if a user enters an ID, OrderHdr and OrderLine that this combination does not already exist elsewhere within the same table.

I have written a query like this:

COUNT OrderLineDetails.id, OrderLineDetails.orderno, OrderLineDetails.orderline
FROM OrderLineDetails
WHERE (((OrderLineDetails.id)=?) AND ((OrderLineDetails.orderno)=?) AND ((OrderLineDetails.orderline)<>?));

I will be replacing the "?" with the relevant form fields from the form, but how do I run the query and store the result in a variable and prevent the user from carrying on until they change any of the fields until there is no duplication.