The 20-degree delta T rule reveals most HVAC problems instantly

Measure the supply air temperature and the return air temperature. The difference (delta T) should be 18-22°F in cooling mode. Below 15°F usually means low refrigerant. Above 25°F usually means low airflow (dirty filter, collapsed duct, frozen coil). This one measurement diagnoses 80% of residential AC complaints faster than any other test. Use a probe thermometer, not an infrared gun.