One of the best ways to find orphan pages on your site is to compare URL lists from full Site Crawls to other data sources.
If you run a full site crawl and download the crawled URLs, then you can compare the URLs from the crawl to other comprehensive URL lists to find all the pages that exist BUT were not found in the crawl or vice versa. You can do comparisons with URLs from: Search Console, Server logs, Site Analytics, and Sitemaps.
If you setup a recurring full site crawl, then you can use the Crawl Compare feature, which allows you to find pages that were not found between crawls over a period of time.