Hi Rob,
There may be another way but try this for the moment:
Open the Items dialog >Tree Manager > Tick 'Group by subtypes > Apply:
Note the listing tree is different and now has CatalogIdentificationpropeties at the top.
Click Save in the Tree Manager to make this layout the default in the active file [not shown as a numbered step]
The only elements listed under this portion of the tree will be those listed in the DataGroup Catalog; including non-building objects that have DataGroup Instance data applied.
Expand this tree to the second level (tedious I know with a long list, there isn't an Expand All option) then select the whole list using the Shift key in the usual way from top to bottom.
Click the Select icon, this will add everything in the list to a selection set.
Use the selection tool to invert the selection, thus highlighting anything that does not have instance data.
This could probably be made simper by creating a macro but not by CoB today!