Start by just using basic viewport size testing, resizing your browser or using a tool like Firefox’s responsive design mode (Ctrl+Shift+M, not sure about the shortcut on Mac OS X).
Nice, I never knew about that feature, thanks! Just tried it out on some of my work, and it's a breeze to use. However, seems that scrollbars behave slightly differently in it. In my case they aren't appearing in certain sub-containers. Have you had any such issues with it?
