Difficult.
I just toggle the "Show only files in Library" checkbox to check what is in the Library or not.
You see, when you use the "Drive & Devices > Explorer" function, and don't check the "Show only files in Library" checkbox, MC temporarily imports ALL files into a database in MC. I believe that is the "Explorer" database. Files that are in your Library are in the Main database.
On top of that, you can't edit the Explorer View, so you can't set rules for display to exclude files that are in the Main database. The best you could do would be to create an expression that sets a value and then sort by that column.
So what you need is an expression, that in English, says;
If the file is in the Explorer database, but not in the Main database, set this column to Yes (or something like that).
I can't find or think of any way to do that.
The closest I can come to what you want is to add a column with the expression:
filekey()
Put that next to the [filename] or [filename (name)] column, and move the [File Type] column next to them. Then sort in descending order on your new column. Files at the top with high filekey() values will probably be files that have been added to the Explorer database just for this session of the Explorer View. You should notice a definite delineation in the File Key numbering between files in the Main and Explorer databases. You will also notice that most files that are not in the Library are non-media files, such as logs, jpgs (Cover Art), text files, and so on.
See the attached image for what that looks like.
Using the modified View you can then scroll through the high value File Key files and look for files that should be in the Main Library, but are not.
