Since Tableau 2018.3, you can preview the different layouts on Tableau Server. Above the toolbar, a new button, Preview Device Layouts, is now available:

When you click on the button, Tableau opens the preview mode where you can select Laptop, Tablet, or Phone to see how your Dashboard renders. For example, if you preview the Dashboard you created before, you can see your special Phone Layout:

This new feature is a great addition to test your different layouts without having to use those devices or special tools.