Message value when remote control switched off

We want to monitor on the ground (laptop), if the remote control has been switched off during the flight. I know, that we can set a failsafe (RTL, Land etc.) when remote is switched off during flight. In case we have a scenario where the copter should not go into failsafe, but continue flying (no auto mission), what value, what status messages shows us that the remote has been switched off / is no longer connected?
We tried reading the RC channels like throttle but they do not change and remain in mid position value. also the RSSI value doesn T help us.
Anyone a hint which value we can check?