Go to Site and Pages, find the page you want to preview and click Preview icon.
Please note that you are previewing this page as admin. So if it is a hidden page or unpublished page, you will still be able to preview it.
To fully test that your page is viewable by public click three dots settings in your browser menu and choose New Incognito Window in the scroll down menu.
Paste your page URL into the browser and click enter to see whether it is viewable or not.
Remember as admin, you have access to see everything, whether it is unpublished or published or visible for logged in or logged out users.