1. From the Page Builder
Simply click the orange 'Preview' button on the toolbar in the footer of the Page Builder.
2. From the 'Pages' tab in the Control Panel
Select 'Preview Page' from the actions menu which can be found by hovering over the cog icon next to any Page.
3. Go direct to the Page's URL
You can visit any Page by visiting it's unique URL in your browser. To get the URL select 'Get URL' from the Actions menu above. This will open a pop-up window with both the long and short URLs.