NXDN TYpe-C systems have a control channel, which is active 24/7. Monitoring that control channel will tell you everything that is going on. NXDN Type-D systems do not have a control channel and all channels in the system are voice channels, with the trunking logic being sent in the datastream of the voice channel during each voice call.
A channel ID is a unique number assigned to each frequency in use on the system. On non-DFA systems the channel ID is sent over the control channel, and one must manually determine the frequency tied to that channel ID. On a DFA system, the channel ID and frequency are both broadcast over the control channel.
You'd be surprised how quickly you can get an answer to that by typing that question in Google.