How to tell if pages have fallen out of index or have never been crawled?

How can I tell that my total number of indexed pages that are growing are not just growing because fresh pages are being indexed and then old pages are dropping out of index?
You can use Google Search Console to check if your pages have fallen out of the index. Go to the "Pages" tab and filter by your sitemap. This will show you the status of all the pages you sent to Google in that map.