Responsive Test Tool is like most responsive testing websites, you can input the URL of the page you want to test into a search bar on the top of the screen. This particular tool has a long list of preset device sizes to choose from. If you need another size, you can choose the custom size measurements. When you want to check a change in the design, simply click on the ‘Check’ button to reload. There’s a button to toggle scrolling capabilities in the testing window and a ‘rotate’ button to check both vertical and horizontal layouts.
The developer behind this tool also created a grid system for responsive websites. You’ll find it in the lightbulb icon to the top right.