How to Detect Subscriber Drops a Message

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You might try this approach:

• Set up a queue inside your node for incoming messages, with some fixed size.
• Callback function for your message subscriber (which runs in its own thread) puts incoming messages into the queue, unless full, in which case it drops the message and increments a count of dropped messages.

• Main loop follows the logic below.

  while (ros::ok()) {
      nh.spinOnce();
  
      if  (droppedMessageCount > 0) {
          ROS_ERROR("%d messages were dropped", droppedMessageCount);
      }
      droppedMessageCount = 0;
  
      if (message queue not empty) {
          take first message from queue and process it
      }
  }
  

This doesn't use any ROS facilities to find out if messages are dropped, but would allow your node to give an indication in the log.