Marking photos that have been sorted into folders

Also nice to know what app you are using, are you referring to photos in Albums or Folders, and do you have a question?

If you are using Lightroom-Desktop (v9.3.1) then you can (in Cloud mode)-

Filter for photos in an  Album-

Click the filter search bar, choose [Album],

Select the Album by name from the list.

If you want to see photos NOT in an Album-

Click the filter search bar, choose [Album], -

Scroll to bottom of Album list and click [Not in an Album]

If you want to see ALL Photos in ALL Albums-

First filter for photos NOT in an Album

Select all photos [Ctrl+A]

Choose [All Photos]

Go Menu > Edit > [Select Inverse]

And create an Album of the selected photos. View the Album.