Can I screw up my board by guessing which RS232 pins are which?
The Raspberry Pi's seriall port pins on the GPIO header are 3.3 volt logic levels direct from the processor.  The processor will likely be damaged if you connect real RS-232 signals to those pins.  You will need an RS-232 interface chip (MAX3232 or similar) to invert the RS232 signals, and convert them to 3.3 volt logic levels.

Then you should use a multimeter to determine which wires in the cable connect to which pins on the RS-232 connector.