Google’s John Mueller said on Twitter that pages can still be rendered by Google’s desktop user agent when Google shifts to mobile-first indexing for a domain.
Yes, we render pages with both desktop & mobile user-agents. Usually it's something like a 2-3:1 split, so when we shift to mobile first indexing for a domain, it's 2/3rd mobile, 1/3rd desktop (the numbers aren't fix, it's just what I commonly see)— 🍌 John 🍌 (@JohnMu) September 4, 2019
Although some pages will be indexed with the desktop user agent, only the content of the pages for mobile will be indexed:
I don't quite understand the question :), but I think you're asking if we index the content from the "non-primary" version — that's not the case. If we switch to MFI, we only index the content from the mobile crawls.— 🍌 John 🍌 (@JohnMu) September 4, 2019
We crawl both primarily to understand the relationship between desktop and mobile versions.— 🍌 John 🍌 (@JohnMu) September 4, 2019
Check your web pages with a mobile user agent
Use the website audit tool in SEOprofiler to check your web pages with a mobile user agent. The website audit tool also enables you to check your pages with a desktop user agent: