What is the right way to audit overdue tasks / milestone?

What is the right way to audit overdue tasks / milestone?

Hey,
I planned a project and start to work, for various reasons there are delays of tasks or milestones. If I change the due date, I will not be able to identify at the end of the project that there were failures in project planning.
Is there a right way to do it?

Thanks, 
Gal