Usual method of reverse engineering is to send stuff to the device and see what it does. And as you can imagine, that is a very tedious job and you miss a lot.
The alternative is to use the software that is written for it and hook a capture device in the serial stream both ways and capture to screen or disk the data transferred.
The device may be processor controlled with firmware. You might be able to decode the firmware.
Possibilites 1&2 would show only the active stuff, not the hidden unused stuff.
Ed Fair
Give the wrong symptoms, get the wrong solutions.