Google has announced that it will change its search indexing algorithms to favor mobile-friendly sites starting on April 21, 2015. This means that responsive sites will see an increase in traffic referred from Google, and non-responsive sites will see a reduction.
In Google's own words, they describe the impact as "significant":
"Starting April 21, we will be expanding our use of mobile-friendliness as a ranking signal. This change will affect mobile searches in all languages worldwide and will have a significant impact in our search results. Consequently, users will find it easier to get relevant, high quality search results that are optimized for their devices."
To test a page or two of a site for mobile-friendliness, Google offers a Mobile-Friendly Test Tool. For a more comprehensive list of mobile usability issues across an entire site, Google recommends visiting the Mobile Usability Report within your Webmaster Tools account.
If you are still operating a legacy, desktop-only web site, we recommend that you begin planning now in earnest to transition your site to a responsive design that works well across all mobile, tablet, and desktop devices. It will be easier to begin this process now, rather than waiting until your traffic and revenues have dropped off.