Similar but different to a @Keselme's answer about checking availableInputs
which did not contain CarPlay for me.

More reliable in my case was checking the audioSession's current route's outputs for port type carAudio:

let audioSession = AVAudioSession.sharedInstance()
  for output in audioSession.currentRoute.outputs {
     if output.portType == AVAudioSession.Port.carAudio {
        return true
      }
    }
return false