How do identify articles that are not grouped

It doesn't appear that List is a searchable field (see https://support.papersapp.com/support/solutions/articles/30000037532-advanced-searching-in-library-) If it were, you could create a NOT search query and save it as a Smart List. Perhaps pass on your request to ReadCube help--I've found them to be responsive.

I do a search in my general library.