How to detect status of switch from serial (or other) port?
AS Spoon suggest, given the available! options, your serial port handshaking lines sound best.

You can use DTR as the common on your switch, connect RTS to the other side. 

DTR = Data terminal ready, should be active when the port is open.
RTS = Request to Send and is an input you can drive active and detect from software.

WHen you get your DB9 cable hooked up use this diagram to find the signals.  I dont know the code for monitoring but you will likely find this in open source. If you plan to use JAVA look at the TX RX lib.

more information about serial ports here...