Freeze cup of water with some red food coloring in it. Turn upside down. If you ever come back to a freezer that looks like it’s covered in frozen blood you’ll know.
I always heard this as an upright cup of frozen water with a penny on top. If you come home and the penny is inside the ice you know the freezer went out.
I think it bears mentioning here that you need to choose a cup that can survive the expansion of its contents. Or in other words, not a glass cup.
That is a much cleaner method
Seems a bit easier than covering your freezer in red food coloring