Handshake? I'll assume you mean, dedicate a digital output, and a digital input, with the output wired to the input, on the remote rack, and continuously toggle the output state. Then monitoring the input would show if comms exist to the rack.
It would probably be better to use GSV to get information about the state of the CNB Bridge in the main rack. Look in the ONLINE MANUALS for ControlLogix, General instructions, Input/Output GSV / Module.