I would think that by now all devices would allow "Always Listening" but the S6 may not. Open Google Now do whatever you need to do to open the menu, for me it's holding the home button, you may have the 3 dots in the upper corner or a menu button. Settings>Voice>Ok Google detection. You may also be able to access this menu from your Google Settings app in your app drawer, but I'm not 100% on that.