I am not sure about doing this in Forms. However in Laserfiche Workflow, once the form comes in you could then make the decision. The only issue is you have to ask all questions that are on both forms, then in the decision path populate the fields according to the lookup.

Thanks Derek, but that won't work in this case.  They want to do the check from the Forms side.  If the submitter says its a renewal but it really isn't, they want to trap that and alert the user before the form is submitted.  Renewals would take a different path than new submissions.

Forms can only perform lookups on an external database, so I would also go the database route. I would have a workflow that maintains a table of the properties that are in the system and updates the table when a new one is added. The form can then perform a lookup on that table.