How to detect if the user of a Silverlight app is logged into server?

I'm looking for a good clean solution to detecting whether a user has been logged out of an ASP.NET MVC application from Silverlight when performing a web request.

The problem is that the website has a Silverlight component that the user could potentially spend a large part of his time in, thus letting him get logged out of th开发者_StackOverflow社区e website. Some of the actions in the Silverlight component triggers a web request to the server (using WebClient), generally getting a JSON result. But if the user has been logged out, the result I get is the HTML for the login page of the system (As the request is redirected).

I could check if the response is a valid JSON result, but if I need to introduce other response types later this will fail. I can also begin parsing the response stream to see if it contains elements from the login page but this seems very inelegant and fragile. Perhaps configure MVC somehow to respond to requests from a specific source by returning a know error response.

EDIT

Using Fiddler I found out that I could look for the 302 response code of the HTTP request. However, it turns out that you can't derive from the WebClient class in Silverlight, so I couldn't easily get to the status code. I considered using the WebRequest class instead but it seems a bit too low level for what I want to do. My current solution is to parse the first line of the response stream.

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If you are using forms authentication with cookies, you could try to check to see if the cookie is present. The following link shows how to access cookies in SL:

http://msdn.microsoft.com/en-us/library/dd920298(VS.95).aspx