Thermometer Trick for Vents

https://cashedux851.dovetailscope.com/posts/comfort-experts-guide-to-indoor-humidity-and-ac-repair-for-north-fort-worth-summers

Thermometer Trick for Vents Quick tip from our house: put a cheap kitchen thermometer in a supply vent after the AC runs 15 minutes, then check the air at the return. We see about an 18 degree drop when things are healthy