There is apparently no way to test whether the current user is a subscriber (student or teacher) or a guest to the current course. There are some workarounds using "Permissions" or assigning all subscribers to groups and then demanding the membership in "any group". However these are not practical and cannot be applied universally. 

Please add a restriction ("Restrict access") to test for guest status (or concersely subscription). This might be an item of "User profile" or a virtual "Group" such as "any subscribed user" or "guest users".

This restriction item would help a lot in designing pages with partially public content.
 

 There is already an implementation of this in plugin database: https://moodle.org/plugins/availability_enrol