You've already got a handy mobile preview of your site in the simulator on the right hand side of your screen. To preview the site for other devices, click on desired preview above the simulator. You can choose a desktop, tablet, or smartphone view.
Once you've clicked on the desired preview, you can click through your site as you would normally if the site were published. Switch between device previews at the top middle of the screen. When you're ready to go back to editing, click on Edit.
