Google has begun sending out 'mobile-first' indexing notifications to webmasters within the last week.
The search engine giant announced that the mobile-first indexing process was imminent just over a month ago, but from the first week of May, SEOs and webmasters have started to receive notifications via the Google Search Console saying their sites have been 'enabled' for mobile-first indexing.
The notification to look out for says 'mobile-first indexing enabled for' and gives the domain name of the site it was enabled for. I.e. http://www.scsmarketingandpr.co.uk