Running a USB stick in "continuous scan mode" and "Rx only" would let you listen to all of the messages being transmitted between the two devices. This is probably easiest done in ANTware II. The biggest caveat is that the only way to tell which "direction" the messages are going is to see the order in which they were received with virtually identical timestamps. In this case, the trainer always transmits first because it is the master channel, and the slave (the PC) is the one back-channelling a message.
