To aid people in understanding how it defines mobile-friendly, Google has created a free Mobile-Friendly Test Tool. Enter a website URL into the address window, and after a few moments analysis the tool tells you if the site is mobile-friendly or not.
If your website fails the Google Mobile Friendly test, it simply means your site does not look good on mobile devices, like smartphones. Google is now elevating sites that pass the mobile friendly test to higher position in the search engine results pages (SERPs). This means a site that is not mobile friendly will drop down in the listings.
The issue of mobile friendliness and the user experience for websites on mobile devices is not only serious, it can be difficult to fix. With the increase in smartphone models, it becomes even more important to ensure your website passes the Google Mobile Friendly test.
The new tool is linked from Search Console's mobile usability report or available directly at https://search.google.com/search-console/mobile-friendly
Update: Google have now given a direct link which doesn't need you to be inside search console and is available here
Trying to optimise websites without addressing the underlying factors like mobile usability is a waste of time. Today, the world is increasingly on mobile. And since Google is obsessed about the user’s experience, this has to be at the forefront of your mind.