Thank you for the post. The way Avenza Maps currently works is by extracting the GPS information from the devices location stream (the data the phone collects) and using it to locate you on your maps. This means that in order to receive readings from an external GPS unit that is connected to the phone, the external GPS has to update or feed its data into the devices location stream for Avenza Maps to access it.
There is not a way in the app where you can determine if your data is being provided by the external GPS unit as it will depend on how the unit works and there is not a direct link established between the external device and Avenza Maps. I would recommend doing two tests: one with the GPS unit connected and one with just your phone and compare the results. Sometimes you can see a difference in the tests and be able to determine where the data is coming from based on its accuracy but there is a lot of different factors that could affect your results so more tests in different places and using different maps may be required to reach a consensus.