On a Mac:
If you're trying to organise your Library and want to find images not in albums, then a simple Smart Album
File -> New Smart Album
Album -> Is Not -> Any
will find these images. As you add them to albums they will disappear from the Smart Album.
On iPad or iPhone, which do not currently support Smart Albums:
Check out HashPhotos
https://apps.apple.com/ie/app/hashphotos/id685784609
It integrates with Photos on your iOS device and adds a Smart Album feature. Using this app a search
!#inalbum
Will find images not in an album
(Hat tip to User ukSamo for the recommending this app.)