Google's John Mueller said this morning on a Google hangout at the 3:40 mark into the video that you can pretty clearly see if your site has been moved over to the mobile first index by looking at your log files.
We know that Google has started the process of moving some sites to the mobile first index but webmasters and SEOs are scratching their heads on how they can tell if that happened.
John said that right now, for most sites, crawling by GoogleBot is done 80% by their desktop crawler and 20% by their mobile crawler. You would see that flip if you moved over to the mobile first index, you would see about 80% crawl rate by the mobile version of GoogleBot and 20% by the desktop GoogleBot.
I think you would probably recognize it in the log files. If you see, if you look now probably something like 80% of the crawling is with the Googlebot desktop and maybe 20% is with mobile with the smartphone Googlebot.He added that we can expect a blog post by the year end to tell us more about this mobile first index and what to expect and how to prepare.
And probably that will shift over and that most of the crawling will be done with the smartphone Googlebot and less crawling with the desktop Googlebot.
So if you like really watch out for your log files probably you can notice that fairly obviously.
Here is the video embed:
Forum discussion at YouTube.