I watched a couple really interesting talks from this past Def Con. In one of the talks, Snoop Unto Them As They Snoop Unto Us, Null Agent points out that all Axon equipment (the company putting tasers on drones) share the same organizationally unique identifier (OUI) and communicate via Bluetooth Low Energy. When you pull your firearm or taser from an Axon holster, it can be set up to signal your bodycam to automatically turn on, for example. So by snooping on the BLE data channels you can look for Axon’s OUI and infer that a law enforcement officer is within your Bluetooth range (max 300ft or so in optimal conditions).
That’s all this script does. If it detects Axon equipment it plays a sound, alerts on your terminal, and logs the MAC address / time of encounter. I run it on my laptop in my living room with a super cheap Bluetooth adapter and I get notified when there are cops outside. Couple this with listening to your local police / public safety radio and you’ll never be surprised by a no-knock again.
If you’re in the United States, the easiest way would be to find them on Broadcastify. That’s assuming that 1.) they don’t use voice encryption, and 2.) someone else is streaming it to Broadcastify.
Your next best bet is to look into Software Defined Radio. To listen in, even on trunked radios, you’d only need a really inexpensive RTL-SDR setup.