Free Resource from Google to Check Your Site’s Mobile Friendliness
If your website is not mobile-device compatible, Google will punish the site’s search engine rankings beginning on April 21, 2015. With the growing use of the internet by users with smartphones, this action is intended to encourage site owners to upgrade their websites.
Google will also add a mobile-friendly tag to search engine results.
If you find problems, you will need to fix them for best performance in Google site rankings. This post from Google has additional suggestions and lists common problems.