You can preview your website directly in the Sitebuilder. There are two Preview icons available to you: a mobile preview and a desktop preview. You can find the icons on the main toolbar in the right-hand corner:
Click on either the mobile preview icon or the desktop preview icon to see how your published site will display.
When you're finished previewing and wish to go back to editing your site, click Widgets or any of the other options available on the main toolbar. This will close the preview mode and return you back into editing mode.