Google announced on Twitter yesterday that they added a new feature to the Rich Results testing tool to allow you to pick if you want to see how Googlebot mobile or Googlebot desktop sees the structured markup on the page.

Here is a screen shot of the tool with the new selector:

Google added this in order to help developers, publishers and SEOs to help them understand which pages are not yet ready for mobile-first indexing. Again, to be switched over to mobile-first indexing, you need to have parity between the pages – the mobile and desktop pages need to be mostly equivalent. Google will begin notifying webmasters that have these parity issues and these updated tools will help the webmasters diagnosis it.

Forum discussion at Twitter.