I checked that when creating a Follow Up activity, it adds a value to civicrm_activity.parent_id so that at least the database knows there is a connection between these, but afaik there is nothing visible on the Activities tab of the contact, nor within either of those Activities if you click 'View'.
While researching this I spotted Find Activities does have radio buttons for is / has a follow up Activity.
Would it make sense for the child and parent Activities to show there is a connection both on the Activity itself and on the Activity Tab
(info based on a 4.6.x version)
EDIT: Spotted this ticket (for civi 2.2.x - wow) that says:
5. Provide link back to parent activity when viewing a follow-up activity Modify ActivityView to check if parent_id is NOT NULL If NOT NULL, assign $parentURL to template - this should be a link to view the "parent" activity. (ActivityView.tpl already has logic to display this link if it's got a value). "Prompted by..."
But I am having no luck seeing this in action on d46 or master. If this did exist and has been lost or removed can anyone confirm if this was intentional? Or is this a Regression?