Well, another simple way....
RS-232 is specified as using up to a +/- 15 volt swing IIRC. If you put a voltage divider inline with, for example, the RI (or DCD, or DSR) handshake lines on the RS-232 port, you can open the port and get a callback on a change of state for that pin. That's about 5 lines of code, and very little hardware.
Might work, might blow up your serial port. You decide.
/frank