Hello Julie
Unfortunately, you cannot directly look for recipients who did not submit your form because no record is created or field updated to indicate that they have not submitted the form. However, you should be able to find this information by making a copy of the marketing list you mailed to, then use Manage Members > Remove Members using Advanced find to remove any members who have a posted form for the form linked in your email. This will remove everybody who submitted the form, leaving only those who have not in the list.