I spend quite a lot of time trying to find if an where fields are pressent on pages. One solution to this problem could be to list All fields in the "Add Field to Page" in design mode, filtering the non added fields on top (maybe using different color scheamas on fields that are present to make this clear). Selecting a field that is present on the page from the "Add Field to Page" menu could highlight (maybe a red frame around it or something) the field on the page that is in design mode.