Flag tasks where Work does not equal Actual Work in time periods prior to Today

I’m using Project professional / Project Server 2010, SP1

Here’s the problem I am trying to solve for:

I want to add a graphical indicator to a task in Project Professional where there is ‘un-used’ work scheduled in the past (in other words, where Work does not equal Actual Work in time periods prior to Today, flag the task).

What is the proper syntax to use?

Dave Shultz, PMP