How do I know that the ViewState has not been tampered with?

This line of copy at the top of a web form's aspx markup is called an @Page directive:

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="Default.aspx.cs"
      Inherits="Damisam.UserInterface.Default" %>

 
 

...and putting EnableViewstateMac="true" inline here ensures that the ViewState has not been tampered with form-submission-to-form-submission.