I dont believe there is a reliable way to track CMR usage, if you have a Hybrid On-Prem solution it uses a specific Meeting Type you can look for, but the Cloud CMR solution does not and can join any Meeting Type.
You could do some testing agains the ClientAgent element returned by the History Service and see if that's unique enough for you to identify your CMR devices, I am unable to get a list of what all valid responses are but look for: Video device,Standalone App Tandberg TelePresence,Standalone App