How can I "listen" to the activity across a wire?
You look at signals in an electronic device by using a tool called an oscilloscope. Obviously there is a twofold hurdle here. One is the acquisition cost and the other is the learning curve to understand and apply the oscilloscope correctly.
There is another type of tool called a logic analyzer that can also be used to capture and display circuit waveforms. When you are working with an unknown interface it is often necessary to deploy an oscilloscope first to zero in on the circuit characteristics and then determine if the logic analyzer can be used.