As of April 21, 2015, Google Search has placed even more importance on mobile friendliness as a ranking signal. This change will affect all mobile searches worldwide and will have a significant impact on Google Search results. Users will find it easier to get relevant, high quality search results optimized for their devices. If your website is not mobile friendly your rankings via Mobiles will be negatively impacted.
That's simple. Google have set in place a Mobile Friendly Test Tool. Click take test which will take you off site to Google's testing page.