I have an Ender3 Pro and on the LCD when the hotend or bed heaters are turned on, the picture of the nozzle turns solid white, or the picture of the bed has wavy lines on it. I'm building a dashboard in Grafana using MQTT, InfluxDB, PostgreSQL, MySQL... etc etc, and I've looked through all the event handlers and API documents and I see how you can send a command to turn them on, but not how to monitor their status. I'm basically just wanting to turn a field green if it's on and blank if it's not.
I don't think there is any marlin response that tells you when it's heating.
I would try something like if the latest temperature > last temperature then it heats up.