Google will implement a change to its algorithm today April 21. They will be expanding their use of mobile-friendliness as a ranking signal.
You can use their tools for Webmasters to review your website. Any site that comes up as not Mobile- Optimized is not indexed as such for mobile searches and gets bumped down from an Search Engine Optimization (SEO) perspective. The tools will also tell you if "Mobile-Friendly" appears next to the website in search results from a mobile device.