This link is to DJI, and the product they make for detecting drones nearby. It sounds as if it works only for DJI drones, but it is unclear. They have specific access to the drones innards to get what they are describing. I doubt that they can do others without access to the other drone's API to interpret the messages floating bak and forth.
"Aeroscope detects UAVs flying within a given area and obtains each aircraft’s broadcast GPS coordinates, flight altitude, speed, orientation, model, serial number, and Home Point. Then Aeroscope sends this information to the remote server for processing and to the display in real-time."
DJI AeroScope is able to identify the vast majority of popular drones on the market today by monitoring and analyzing their electronic signals to gain critical information, allowing users to protect the integrity of their flight-sensitive environment.
www.dji.com