The most useful log is emonhub.log.
You also need to log the status of the following services, emonhub, feedwriter and mqtt_input.
A “top” printout to show RAM usage by process and cpu usage might be useful too.
[edit - also if you must use a watchdog, you really should check emoncms directly rather than openhab. you are not checking to see it the data is updating, you are checking to see if the data is being shared with openhab. emoncms and emonhub might not be failing if you are checking openhab. Also using an emoncms feed api would allow you to check the time the feeds were last updated NOT the last time the value changed. very different things, a static value may never change even when it is successfully updated every 5secs.]