how to track unapplied employee advances?

My fellow NS users,

We give our employees advances (for example, for travel) and register that into NS with "write check" specifying the NS generated account "advances paid"

The employee is expected to enter an expense report. The advanced amount will then be applied and the difference between the actual expenses and the advances paid will be settled. However, not all employees enter expense reports eventhough they should. (specifically when the advanced amount is greater than the actual expense)

We need to generate an overview of all paid advances for which no expense report has been filed yet, i.e. an overview of all unapplied advance amounts. What is the easiest way to accomplish this?