You can self solve this with a fridge thermometer, e.g. https://www.amazon.co.uk/Thermometer-Refrigerator-INRIGOROUS... or if you’re into that smart home biz, I also have an Aqara zigbee temperature and humidity sensor for automated alarms.

EDIT: sibling comments about irreversible temperature monitors are also a great idea I hadn’t thought of! Time to buy some of those too