How to determine security zone used to access site?

I'm looking for a server-side property in ASP.Net that will tell me what browser security zone is being used to access my web page?

 I need to provide the user with instructions for setting up CASPOL security for an embedded winform control and I would like to automatically determine whether the user is running in the Intranet v. Trusted zones.  So far, I haven't been able to spot a property on the request object or elsewhere that tells me what zone is being used to access my site.

Thanks!
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