To preview a blog post, it will first need to be saved by either using the grey Save as Draft or the green Publish button on the bottom right of the page. (For instructions on how to create a blog post please refer to this link.)
Once the blog post has been saved a green Preview on Website button will appear at the top right of the screen. Upon using this button a new window with the preview of the blog post will open.
NOTE: When using the Preview on Website button in a blog post, the system is choosing an arbitrary date for the preview. Once a blog post is published it will show the date it was published, if not set differently in the Publish Blog Post on (optional) field.