How to locate where a Master slide layout is used in a course.

I delete slides one by one or by scene and go back and check the master slide to see if its unused (making sure I took a backup copy first). Painfully slow process, there really should be a list you can generate/see where all slide templates used are listed in one place. Annoyingly in one project I found a slide template used by one slide, it was the 'Draw from question bank' so couldn't be removed.