DE-9 monitoring solutions?
The advice in comments is good that for a start you should use a multimeter to confirm it is a standard RS-232 connection. Check for something between +/- 3V to 15V between pin 5 (signal ground) and pins 2 or 3 on the DE-9 connector which are either receive or transmit depending on whether the end you're checking is configured as a DTE (data terminal equipment) like a PC or DCE (data communications equipment) like a modem.

Even if you don't measure those voltages sometimes OEM equipment will use an RS-232 like protocol that only swings from 0-5V to save the cost of generating a negative voltage especially if it's only for use over a short distance between their own products. In practice that often works with a PC and USB to serial converters as well.

It if does looks like it may be a standard configuration here's a cheap adapter I made to intercept serial data a while back. Just use a pair of native serial ports or USB to serial converters along with a free terminal program like RealTerm. Depending on the complexity of the protocol you might also get away with monitoring a single side of the communications at a time with a single port.