Lightroom: How to identify pictures not assigned to an album?

I have already uploaded 12k pics into the cloud, mainly by importing Classic catalog exports. I assigned most of the imported pics to an album, but I'm struggling identifying the orphains who remain unassigned and do not belong to an album. I might miss a function, but cannot find anything in the support files.
Having this said, the filter function deserves a significant update to eliminate these uncertanties.

Thanks
Torsten