He could

Place a pot of fleas under a microscope.

This would work as

Fleas would die at 32 degrees Fahrenheit. So if he wants to check if it went to that temperature he can look at the microscope to see if all the fleas are dead.

You could also

Place a small animal in there that would die at that temperature, like a type of goldfish or something.

He could

Place a pot of fleas under a microscope.

This would work as

Fleas would die at 32 degrees Fahrenheit. So if he wants to check if it went to that temperature he can look at the microscope to see if all the fleas are dead.

You could also

Place a small animal in there that would die at that temperature, like a type of goldfish or something.

True, this could be seen as animal cruelty, but when the world is in danger of a zombie apocalypse, I don't think anyone will complain.