How do I know if my 9-pin serial cable is a null modem?

I have a 9-pin serial cable that I found in a box of computer cables that I have. I would like to use it to connect to an Apple ImageWriter II printer with my PC. I have a gender changer and an adapter cable as well to connect it to the ImageWriter since it uses 8 pin DIN-8 serial. I also know that the whole process needs a null modem somewhere, and I was wondering if my female-female serial cable is a null modem. How can I find out? It has absolutely no markings on it anywhere, except the RU and SA logos on the ends.