I have done some digging and I think I have a solution for you. Work orders created by self service have a blank value for the OpenedBy field because they were not created by a technician. You can set up a grid view to show all work orders with a blank OpenedBy field and see all of these work orders.
One thing you might run into if you use other modules like Software License Management, is you may have other work orders created by the system with blank OpenedBy fields. If so, add a second parameter to the grid view to show work orders where the Requestor is NOT blank. All work orders from Self Service will have a blank OpenedBy field and a Requestor assigned to them. Hope this helps.