Context: I don't use m4a or similar files so don't know how a sidecar behaves. All my files can store images internally, but I don't want this. My files either have external cover art, or none at all. If sidecar files do something different, this method might not help you.
In my situation, the path to a track's external cover art is stored in field Image File.
To identify tracks that have a path to an external image, I have a Smartlist with expression -[Image File]=[] which means include only if field Image File is not empty. Therefore, the playlist does not show tracks that have no external image file. (The opposite behavior would be useful to show what tracks "need" images, but that's a different project.)
To see where the external image files are located, I add column Image File to the playlist view. I sort and group on it to see file path patterns and the various locations of image files -- the usefulness of doing this depends on how the library is set up. (I have all my cover art in one folder.)
I created this special "Has Cover Art" smartlist years ago, and use it periodically to check and correct cover art link problems.