Mobile-First Index will Support AMP Pages
AMP pages will be used in the mobile-first index if they are set up as separate mobile URLs, or if the AMP page is your canonical page.
Use Redirects to Remove a Separate Mobile Site
Primary Content’ Should be the Same on Mobile and Desktop Pages
Google expects the visible Primary content to be the same on mobile and desktop pages, including AMP pages, but the navigation can be different.
AMP Sites can Also be the Mobile Version
You can reference an AMP site with both mobile rel alternate and rel AMP HTML, however the AMP page will become the primary version when Google moves to mobile-first indexing.
Structured Data and Hreflang Need to be Added to Mobile Pages
When Google moves to mobile first, the rel alternate and canonical tags won’t need to be changed, but the mobile pages will require dedicated structured data and hreflang tags.
Links on Mobile Pages Will Be Used For the Link Graph
Mobile-first indexing will use the links on your mobile pages for calculating the link graph.
Use Fetch & Render using Smartphone User Agent, Mobile Friendly Testing Tool, and AMP Testing Tool to test for Mobile-first Indexing
The Fetch and Render tool set to use a smartphone user agent, the Mobile Friendly Testing Tool, and the AMP Testing Tool can be used to show you the content Google will use for the mobile-first index.
Separate AMP Pages Don’t Need Hreflang
If you have separate AMP pages, you don’t need to add hreflang. Implement in on the main desktop pages, the same as a dedicated mobile site.
Separate Mobile Pages May Appear for Site: Queries
Sometimes you may see your mobile URLs appear for site: queries, but they might not be indexed. Check the cached version of the pages to see what content has been indexed for those URLs.