Google has been hinting at making mobile friendliness a ranking factor for several months, and just recently announced that it will indeed be a ranking factor. This means that sites that are not mobile friendly will see a dip in search result rankings and will be marked as not mobile friendly.
There are three quick ways to determine if your site is mobile friendly. One way is to simply access the site from your phone. If it is friendly, the site will either load with a mobile response site design or a mobile only site.
If you are on a desktop, and want to quickly check if your site is responsive, simply adjust your window size down. If the design on the screen adjusts with the size of the window, then you know that the site has a mobile responsive design.
The way that you can be most sure of your site’s mobile friendliness is to actually test it with Google Webmasters Mobile-Friendly Test. There are some design elements that Google considers non-mobile friendly that you may not pick up on yourself. This tool will tell you how Google is reading your site.