check out http://www.aggsoft.com/rs232-pinout-cable/pinout-and-signal.htm and
http://www.lammertbies.nl/comm/info/RS-232_flow_control.html There are few hardware signals.

The trick is see weather you PC program has direct access to theses RS232 hardware signals.

Some programming languages don't allow lower level hardware access to the RS232 signals.

For C# you can use the SerialPinChange Class on the SerialPort object.
See here http://msdn.microsoft.com/en-us/library/system.io.ports.serialpinchange.aspx

IE you can test on the PC weather a PIN has changed state for the following signals (PC is DTE)
CtsChanged The Clear to Send (CTS) signal changed state. This signal is used to indicate whether data can be sent over the serial port.

DsrChanged The Data Set Ready (DSR) signal changed state. This signal is used to indicate whether the device on the serial port is ready to operate.

CDChanged The Carrier Detect (CD) signal changed state. This signal is used to indicate whether a modem is connected to a working phone line and a data carrier signal is detected.

Ring A ring indicator was detected.

Break A break was detected on input.

Joe