Verify who is ASP.NET Logon User

Usually I use Request.LogonUserIdentity.Name to check ASP.NET Logon User. It works for all types of users.

      System.Web.HttpContext.Current.User.Identity.Name
or
      Request.ServerVariables["AUTH_USER"]

are only for Windows Authenticated user. For Internet users (anonymous guest), empty string is expected.

Request.LogonUserIdentity.Name will show you if the user is anonymous.