You can parallel another adapter on the IN or OUT ports, and connect that adapter to a separate laptop. If you are just running OpenCPN on the laptop, you can look at the datastream on the diagnostic window. If you are using Linux on your laptop, you can issue 'cat /dev/ttyUSB0' (or whatever the interface gets registered as) to see the data stream.
You could also do it with an oscilloscope, but you won't be able to see the actual data that's being sent, just the waveform.
You could also do it with an oscilloscope, but you won't be able to see the actual data that's being sent, just the waveform.