Tasks tab in the Status sheet, select the task you want to inspect in the task tree, and open the Task Details pane. field "Last execution" will specify the date+time of last execution, and field "Started/Scheduled" will specify the trigger that launched the last task execution.
For historical triggers:
Select the task you want to inspect, open the Task History tab, select the run from the list and click the link. The Task Log will be shown. On the first line, scroll to the right until you encounter tag 'Triggered by'.The following text will detail the trigger type
Trigger definitions can be found in the Trigger.xml file in the QVPR.