Point it up at the zenith of a clear, dark sky on a warm night. Brrr!

Inexpensive IR thermometers ignore water vapor (so people can use them
in boiler rooms full of steam), so this might also work on a warm
summer day.

Don't misunderstand. You're reading the near-absolute-zero temperature of
outer space, through a thin veil of warmer air. The integrated temperature
is still below zero even on a warm night. The clear night sky is a cold
window.

"Ignore water vapor" is fantasy. Blackbody radiation is the same whether
it's a gas, liquid, or solid radiator.