Calling an Action on Filtering

[Andrew]

I'm using the MVVM approach.  I have a RadDataFilter that binds to an object in my ViewModel.  A RadGridView binds to the FilteredSource of the DataFilter.   Below the grid I have a labels that is the count of records shown in the grid.  When someone filters, I'd like to update the count shown below.  I've tried setting a breakpoint on my ObservableCollection in the ViewModel, but the SET on it doesn't appear to be called.  I've tried binding to the Filtered event of the GridView with no luck.  

Is there any way I can call an action in my ViewModel when someone makes changes to the DataFilter?