Google has created a Mobile-Friendliness tool that will give you a ‘yes’ or ‘no’ answer on a per-url basis. Pages are evaluated individually, so another quick way to get a sense for how your top pages perform is to do a “site:” query for the domain in question on your phone. That will allow you to see all the pages indexed to the domain, and evaluate which ones are considered Mobile-Friendly and which are not, without having to submit them to the tool one at a time.
Google has been clear that Mobile-Friendly test results are binary, meaning that your page is either Mobile-Friendly or it is not. There is no 50% or 70% Mobile-Friendly result possible—no middle ground. They have also taken care to specify that Google’s Mobile-Friendly evaluations are somewhat instant, implying that there is no proving-time or “sandbox” associated with the tag, but this could be somewhat misleading. There may be no intentional time-delay before a page is awarded the Mobile-Friendly notation, but it will only change after a crawl of the site indicates that the page is now Mobile-Friendly, so it isclose to instantaneous if the pages are getting crawled on a very regular basis.
We have found that the tool result does not necessarily match up with what we are seeing on our phones. We have occasionally also noticed that sometimes two pages in the same page template will perform differently, even though the content that changes between the template is primarily text. Both of these variations could simply be an indication of real-time delay between the tool and the crawler—the tool does an ad-hoc check on the URL to assess mobile-friendliness, but if the bot has not been by the site to evaluate its mobile friendliness recently, then the page in question would not yet have the Mobile-Friendly designation in the SERP. With this in mind, remember that when you are updating a page, and pushing it live for testing, you must use the tool to see if the update has been successful, until the site is re-crawled. This also means that once you see success in the tool, the best way to get the Mobile-Friendly designation to show up in the results faster might just be to push a sitemap in Webmaster tools, and try to trigger a fresh crawl.