In our RDM controller, we chose to use the SET:IDENTIFY_DEVICE (OFF) PID to 'ping' known responders to see if they were present on the network. This is a mandatory PID so there is no ambiguity about the response from the device. It also has the happy side-effect of clearing any stuck identify states in the responders. Of course this method might not be suitable for all controllers, but it worked for us.
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