Google has mentioned about the two most common issues it sees when evaluating sites for mobile-first indexing. As mentioned earlier by Google, sites are only moved to mobile – first indexing when they are ready. Nearly over half of the pages shown in global search results have been moved to mobile – first indexing up till date.
Site owners would certainly get to know when their site has been moved over. This is because Google will send a notification via Search Console. If your site has not been moved to mobile-first indexing, it could be that Google has not gotten around to evaluating it yet.
In case your site is not using responsive web design, these two issues might be getting in the way
Missing of structured data on mobile pages – If the structured data is used on the desktop version of a page, it should also be used on the mobile version. This is significant because with mobile-first indexing, the user would only use the mobile version of your page for indexing, and might miss the structured data.
Alt-text is missing from mobile pages – If alt-text is used for images on desktop pages it should also be used on mobile pages. Alt-text makes it easier for Google to comprehend the context of images used on web pages.
Tips to check for mobile – first indexing manually
You may use Google’s URL inspection tool. This would allow the site owners to check how a URL was last crawled and indexed.
You may run a URL through the tool. Usually, just the homepage is enough. But, you may check for “Googlebot smartphone”.
For more tips on this, you may seek help of SEO professionals.