Hi Ben
You can determine from which site the person submitted the form by creating a series of advanced finds (one per unique site) for leads who have an associated form-type page view where the entry page for the page view's visit contains the domain of the site you are interested in. This will indicate that the lead accessed the form on that site, but not necessarily that they submitted it. You can add another check to the advanced find to see if the lead also has an associated posted form record for that form.