On June 26, Google announced that it made a “few tweaks” to Google Search Console to help webmasters and SEOs understand how Google is indexing their sites. With these tweaks, you’ll be able to see if an when Google switched a site to mobile-first indexing.
Under settings for a property, Google can show you what Googlebot is indexing the site (Googlebot Desktop or Googlebot smartphone). Not only that, if it is Googlebot smartphone, Google will give you the exact date it switched over to smartphone from desktop crawling.
Google will show you the useragent (Google desktop or Google smartphone) being used for reporting within Google Search Console’s various reports. The indicator will be displayed in the top right under “primary crawler.”
Google will even show you within the various charts when the site was moved from desktop indexing to mobile-first indexing.