The app itself has a built-in viewer that can be used to the contents of the backup file. This can be accessed using Menu > View backups.
To view the backups outside the app or from a computer, use the online viewer on our site.
Note that the online viewer processes the whole backup file within the browser on the computer itself and can have troubles loading very large backups, especially if they have too many images/videos etc (MMS messages). If you find that the viewer gets stuck or shows errors about memory issues, restart the browser and then make sure the checkbox to “Load images and other media” is disabled before selecting the xml file to view.