On July 1, Google announced that the company has implemented Mobile-first indexing by default for all new sites.
The company recommends to all owners of old sites that Google has already indexed to check whether Mobile-first works for them in Google Search Console. now every webmaster can check it personally.
If in the field Indexing crawler Googlebot Smartphone is located, which means that the site has been transferred to the Mobile-first index and this bot is considered the main one. To check use URL validation tool… In addition, now in Search Console you can also see the date of transition to mobile-first.
Source: Google Blog