It doesn't matter if your website has been published already, or it's just a draft, you can preview it from your Dashboard.
Hover the mouse over the ellipsis (…) button. If your website isn't published yet, you'll see the Website preview option. If your website is published, click on the View live website.
Alternatively, you can preview your website – published or not – within the builder itself when you're working on your website. Simply click on the Preview button and your website will reload in the same window.