To preview your message before sending, you can either use the button in the bottom-right corner of the message editor screen, or the "Preview Message" button next to the small preview on the Message Details screen. If you do not see the preview button in the message editor, make sure that any changes you've made are saved.
Any dynamic content configurations will be shown in the preview. Keep in mind that, if any new content is published before the message is sent, the preview may not match what is eventually sent to recipients.
In the bottom-right menu, click the mobile device icon to preview how the message will look on multiple screen sizes.
Use this feature to view the responsive nature of the message in action, as well as test any elements that are set to be hidden on mobile.
You can also use the "Preview As" functionality to test any merge tags that you have included in your message.
Use the filter field to select a recipient from the mailing list, and you will be shown the message with any merge fields filled out as it will be sent to that user. The subject line is shown above the Preview As field, so you can see any merge tags used there, as well.
Once you have finished previewing the message, click "Edit" to return to the message editor, or "Message Details" to return to the Message Details screen.