It is worth noting that the Networking Event does not get triggered in clients when the server disconnects, and that neither of the network_type_* events will be triggered in clients when the server they are connected to disconnects, even if the connection is TCP based.
The implications of this is that you have no choice but to write your own methods for detecting whether the connection to the server is still open. GameGeisha's method is the easiest method I can think of.
Thank you I somehow missed this in the manual. This behavior seems very odd to me, however... this is the first time I have run across a tool or programming language in which the client did not know if the connection to the server had been lost!