How to detect client-side if the server disconnects me?

You can simply do a callback just before closing the callback channel telling the client you're closing the channel.

So just before this line of code:

((ICommunicationObject)client.CallbackChannel).Close();

I went with the Abort-approach. It seemed like the most logical method and it works really well. The client gets notified with the Faulted-event on the callback-instancecontext.