Idk, usually I can just tell when I get hot or cold if the central air isn’t working myself.
It’s a thing for commercial buildings with stuff high up snd you can’t feel the draft from without a ladder, but not in houses or trains or anywhere else Hollywood slaps them. It’s not a thing, why are so many people trying to claim it is? Lol
Let’s see yours on your vents if it’s a thing then.