There’s plenty of efficient software and hardware serial port sniffer tools able to meet your most specific needs. However, considering that a software solution like Electronic's Team Serial Port Sniffer is much easier to install and configure than any hardware serial monitor, it seems to be the most optimal method to monitor RS232 communications. In addition, pricing is usually a bit higher for a piece of hardware (supplied with a controlling app, cables, and connectors) than for a standalone sniffer program.