Listen before talk (LBT) of SI4463
The datasheet says: "The latched RSSI values are cleared when entering RX mode so they may be read after the packet is received... or after dropping back to standby mode". This suggests use of the following functions from the RadioHead library:

void RH_RF24::setModeRx()
void RH_RF24::setModeIdle()
bool RH_RF24::get_properties(...)