Ultimate Permission Manager adds a very handy macro to Confluence that is able to list every user who has View permission to the page containing this macro. The output can be filtered dynamically and the number of rows (users) displayed at one time can be customized. The macro evaluates both group memberships and page restrictions (the later can be disabled)

To add the Who Can View macro to a page, open the Macro Browser and search for "Who Can View":

Who Can View macro embedded in a Confluence page: