AFAIK you can't know that on the serverside.
You'll have to make a call from the clientside to notify the server that the response has been received.
The easiest way for doing that is by making a remote call with Seam remoting.
And most probably, you'll also have to make a timeout mechanism on the serverside if the clientside doesn't respond in a certain amount of time.
thank you for the hint serkan!