UPDATED: Music Folder Files Not Added v4.5
Music Folder Files Not Added will list the file paths of the files in your designated "iTunes Media" folder which are not in iTunes' track library. Additionally, you can select a different parent folder and its contents will be compared to the iTunes library. Includes options to Add a selection of found files to iTunes, move them to the Trash, and export a text file listing the file paths.
This latest version accommodates some changes in iTunes 12.2 and provides preliminary support for OS X 10.11 (El Capitan) beta.
Music Folder Files Not Added is free to try for 10 days, $1.99 thereafter. This is a free update for registered users.
More information and download is on this page.