Currently Silverlight 4 is in beta. The latest release version is Silverlight 3. You can code up a test application. A good programming language for the test app is C#. This test app can send messages to the app you are testing. These messages interrogate the application for its current state.
By default a Silverlight app will take up the whole screen. So you need to make sure your test app can stay on top, or share the screen real estate with the app being testing. The use of Local Messaging is a simple way to communicate with the app being tested.
Simplicity is the main idea here. You do have to modify the app being tested. This feels a little more like unit testing to me. However it is a light version of it. There are alternatives to testing Silverlight apps. Perhaps I shall delve into those other techniques in a future post