A task will have a icon, while the subtask will have a icon. Additionally, in the details window, a subtask will not have a Subtask tab. Instead, a parent task will be listed in the details window below the Tags.
Users will need view permissions in tasks to perform this action. Depending on the user's project permission settings, they may be able to view all project tasks or just a subset. Watchers specifically added to the task will be able to view it.