I have remote gauges with alarms in my freezers, but I'm in my fridge every day, and nothing beats a simple analog gauge.

In the freezers I also employ either the "freeze some ice cubes and put them in a baggy" or "freeze a small jar and put a coin on top" methods.

If you see the ice cubes have melted and refroze, then trouble. If the coin is not on the top of the jar -- same thing. Fail proof methods.