Monitoring serial communications between two PCBs?
This is the kind of thing logic analyzers were created to do. But you could try using an Arduino too in the beginning. To ensure you're starting from similar states, it might be best to first try listening when you turn on the unit. Try something low like 1200 baud then move up to 4800, 9600, etc. There is a list of common ones at http://digital.ni.com/public.nsf/allkb/D37754FFA24F7C3F86256706005B9BE7
It's also most commonly 1 stop bit, no parity, and no hardware handshake. But also have a look at what voltage level (RS-232, TTL 5v, TTL 3.3v, etc) it's transmitting at too.

If you have a logic analyzer like a Saleae Logic (they make very nice ones with good software) it even has an 'autobaud' that could try to detect what baud rate the input is operating at. Then you can try and monitor both sides and see what one side is saying to another.
It may be binary data that you'd have to try to parse out, but you'd be surprised how often it's just readable ASCII too.

Good luck!