To preview your site, click the View my site button in the top of the admin. This will then prompt you with two further options. To view your site as a paying member or non-member. Viewing as a member will give you access to the members-only sections. Viewing as a non-member will show you what non-members see when visiting your site.
When you view as a member, you'll be able to select specific membership levels. This will allow you to see the effect of any content restrictions based on membership level.
Nothing happens when I preview my site
Please see this article if nothing happens when you preview your site.