Connect the Amiga to the PC with your nullmodem cable and run a terminal program on both computers (for example HyperTerm on Windows and NComm on the Amiga). Enter some text on one end and check what is received on the other end. If you see what you entered, everything is fine. If you receive garbage, either the parameters (baud rate, start-/stop-/parity-bits) aren't matching or the hardware does not work correctly. If you receive nothing, it's surely the hardware.