A nice preview helps you get a feel for what your promotion will look like in inboxes. When you’re in the composer, click on the link in the upper right corner that says Preview.
You’ll arrive at the Web View of your promotion.
On the bottom of this same page, click on your promotion title to reveal a menu of all past promotions.
Click on any promotion title in the list to preview that promotion. You can toggle through all of your promotion previews on this same page :) There’s also the option to share any promotion using your Share URLs on the lower right, and get back into edit mode with the edit option on the lower left.