If you want to preview your notification bar, then there are various ways you can do this.
First, you will need to set up your bar and save the draft or publish it. Note that once you do this, you will no longer be able to change the bar type.
You will now see a preview appear on the settings page. It will appear at the top or bottom, depending on the setting you have selected.
You can also launch a Frontend Preview, which allows you to see how your bar will appear on a particular page (or all of them). Update to save any changes, then click on the Launch Frontend Preview button on the right of the settings page. The preview meta box also provides instructions on how to view the bar on other pages as well.