Mobile theme responsive?

Is the mobile theme responsive or is it using the same theme as the desktop. I seem to be having a few issues when I check the mobile site and a fellow colleague asked if it was a responsive mobile theme or if I was using the desktop theme on mobile. I of course have no idea hahaha. So if there is a responsive mobile theme, is automatically activated or are there steps I need to take to ensure the theme Will properly adapt the site display to match that of the mobile device. Thanks