The radios come from the factory unprogrammed. In fact, when you turn on a brand new on out of the box, the display will show "UNPROGRAMMED". They require programming for the frequencies the owner is licensed for.
If you know what frequency they are programmed for, they may be able to be monitored.
-There are scanners that will decode NXDN/NexEdge
-The radios come from the factory with included encryption, and it doesn't cost anything to turn it on. If the radios you want to listen to are encrypted, you can't/won't be able to decode audio.
-You can purchase a Kenwood or Icom NXDN capable radio and program it for receive only, however, if the radio is running encryption, you still won't be able to hear audio. If it's a NexEdge trunked system, it will not decode the trunked system traffic unless your radio is legitimately on the system (as in programmed for it by the system administrator)
-If you want to listen to NXDN/iDAS/NexEdge, your best bet is to get a scanner.