How can I track the completion of a task as it relates to the due date?

Tasks can be tracked within the Task Viewer but the best way to monitor how the completion date of the task compares to the original due date of the task can be done via the Task Completion Report.

The Task Completion Report is a standard Status Report that shows completion dates of tasks.

Clicking on Reports > Standard Reports > Task Completion Report will allow you to set the parameters for the report including what type of tasks you want to see, for what roles and users, the task status and the task list names.  

This report shows the requested tasks, their associated owners, original due date and completion dates. See example below: