How can I see if a task's executive viewed the assigned task/ subtask?

How to complete a task? Who can complete a task? How to find the list of tasks of a project (or account)? How to edit a task? What other actions can be performed with a task? Who can mark task as done? How to organize tasks by labels? (Where to configure labels and who can do that?) How to change the time frame of a task? How to configure a customized list of tasks and save it to favorites? How can you use the statuses of tasks? How to set priority of tasks? If I change a task or a subtask, will all of the subscribed participants be notified about that? How to enable/disable display of subtasks in the list? How to launch a task's timer? How to add a file to a task, subtask, or a comment? How to view the history of events in a task? How to change an executive of a task? Bulk task operations (copy, transfer, delete...) How to group tasks by the projects? What can be done with the list of tasks (export, print send to email)? Can only tasks with a certain label be shown? How? How do I see tasks for today? For the next 3 days? Overdue tasks? How to set name templates for recurring tasks? How to repeat a task with nested subtasks? How to nest/extract a subtask to/from a subtask? Second level subtasks. How to view the work load by people? How to see who has tasks overdue and how to see the list of all overdue tasks? How to view completed tasks of a project? How to reassign tasks between the project participants? How to copy tasks with dependencies from one project to another? Can there be several executives assigned for one task? How to check a task before closing it? How to copy a task with dependencies within a project? How to create & save custom filter on task list / reports / Gantt chart How to find a task by ID?