Gysbert is right... there is no inherit property to distinguish whether it's User or Sytem variables. However, if you can follow a standard naming convention while creating the variables then you can use string/ pattern search to identify whether its User or System variable. And this is good practice when you have more than handful of variables. I always follow a standard naming convention, something like this:
Once you follow the standard naming convention then you easily call your variables by using below script.
Set Vars = ActiveDocument.GetVariableDescriptions
For k = 0 to vars.Count - 1
Set v = vars.Item(k)
IF Left(v.Name, 2) = "v_" Then
I hope this helps!