Website & Mobile:
To make a post public or private, select the appropriate button when creating a new post, editing an existing post, or finalizing a draft. This buttons appears on the bottom of the page as:
To identify if your post is visible to everyone or set to private, navigate to your feed/post and look for the small orange eye icon. Posts with the orange icon are private and only visible to logged in users while posts without the icon are public and visible to everyone.
To change it to public or private, click the 3 dot 
option on the post and select, Edit. From here, you can modify the privacy.