Viewport Resizer is a tool to test any website's responsiveness. Just install the extension, go to the page you want to test and check all kinds of screen resolutions of the page. The smartest way.. ViewportTester displays the viewport width and height values straight from within CSS media queries.
I was never able to get it working with 100% width in its containing element, but things went pretty easy once I switched to vw units (a unit that is 1% the size of the viewport). If you're aiming to do the full width of the viewport anyway, 100vw is a fine alternative to 100%.