Though there is no API so far also I could find out. I have tried a workarond. From the server side PHP I am sending an FCM message. Let me present how I have handled this in PHP.
$fcm_return = fcm_message_send($message);
$fcm_return_JSON = json_decode($fcm_return, true);
echo $$fcm_return_JSON['success'];
As I am sending a message to one device, I am comparing return with 1. Based on this you could further implement your logic.
