The SlideIndex of the current slide
Displays the SlideIndex of the current slide in Window 1
MsgBox ActivePresentation.Windows(1).View.Slide.SlideIndex
You can locate the number of the current window with something like:
For X = 1 to ActivePresentation.Windows.Count
If ActivePresentation.WIndows(X).Caption = ActiveWindow.Caption Then
WindowNumber = X
End if
Next X
See How do I use VBA code in PowerPoint? to learn how to use this example code.