Here is the example from help for using the Application Level SysVarChanged Event. The system variable you need is WSCURRENT.
Public WithEvents ACADApp As AcadApplication ' Use with Application Event Examples
Sub Example_AcadApplication_Events()
' This example intializes the public variable (ACADApp) which will be used
' to intercept AcadApplication Events
'
' The VBA WithEvents statement makes it possible to intercept an generic object
' with the events associated with that object.
'
' Before you will be able to trigger any of the AcadApplication events,
' you will first need to run this procedure.
' We could get the application from the ThisDocument object, but that would
' require having a drawing open, so we grab it from the system.
Set ACADApp = GetObject(, "AutoCAD.Application.16")
End Sub
Private Sub ACADApp_SysVarChanged(ByVal SysvarName As String, ByVal newVal As Variant)
' This example intercepts an Application SysVarChanged event.
'
' This event is triggered when the value of a system variable is changed.
'
' To trigger this example event:
' 1) Make sure to run the example that initializes
' the public variable (named ACADApp) linked to this event.
'
' 2) Change the value of a system variable in AutoCAD.
' For example: Type GRIDMODE on the command line and toggle the grid display on/off
' Use the "SysvarName" and "newVal" variables to determine which
' system variable has changed and its new value.
MsgBox "The system variable " & SysvarName & " was just changed to: " & newVal
End Sub
