How do you verify that your hand is working? Your brain sends signals that propagates to your arm telling it to move, and if you your brain gets the feedback it expects, its working. Its all signal propagation. For any problem you need to solve, you can only do one thing - send a signal, and hope that the communication lines are working correctly execute on what you want.

Yeah but how do you measure it between two very different trees. What are we measuring and from where. Electric current?

"Microsatellite DNA markers were used to identify tree and fungal genotypes isolated from mycorrhizas and for comparison with reference tree boles above-ground." https://besjournals.onlinelibrary.wiley.com/doi/full/10.1111/1365-2745.12387#main1