How do I get the Windows user so I can display it?

I'm starting a new Web Forms app that's meant to run only as an Intranet application. We're a Microsoft shop, and all users log into our company's Active Directory. I'd like to be able to display the user's name in a small label (scan) on the page via the Site.Master file. I've searched online and it looked as though I could use this line:
string strName = Request.ServerVariables["AUTH_USER"]; However, when I turned Trace on for the Default.aspx page, I discovered that the server variable AUTH_USER was empty, so that doesn't look like it's going to work for me. So how can I get it done?
I'm using .NET 4.5 in VS 2012.