A recent Google algorithm update which was released on the 21st April 2015 to target websites which are not mobile friendly has not affected websites which were designed to be responsive. Usually Google does not warn in advance of algorithm changes which may affect websites but the mobile search changes were announced earlier in the year to allow website owners to make any changes as required in anticipation of the update.
Businesses who were fortunate enough to have created their website to be responsive would have sighed in relief as it meant that any search traffic which their website receives from mobile requests would not be impacted. There is also no affect to search traffic generated from desktop requests even if your site is not mobile friendly. So you may ask yourself if there is any need to change your site to a mobile friendly website. If you were fortunate enough to use a reputable SEO consultant or agency then you would have expected that your mobile search traffic for the website was reviewed when the proposed changes were initially announced. If you have noticed a drop in traffic from mobile searches or indeed have recently noted that your keywords have dropped in rankings when you search from a mobile phone then the chances are your site is not responsive.
For those businesses who do not have the technical knowledge to transfer their website to a responsive design or it has not been explained if their site is mobile friendly then it is worth just searching for your site on a mobile device. Search Google for a known keyword which your site normally ranks for and it will probably now have the term mobile-friendly displayed in the search description if it is. Websites which are not mobile friendly are more difficult to navigate around as the buttons are close together or the font may be smaller or the site may not fit on the mobile screen without sliding and manipulating it around the screen to view all parts of the page.