Understand the principle before the equipment
Temperature change across HVAC equipment is one way to evaluate heat transfer. In heating, technicians may measure how much warmer the supply air is than the return air. In cooling, they may measure how much cooler the supply air is than the return air.
These numbers are not universal pass/fail values by themselves. They depend on airflow, equipment capacity, indoor conditions and manufacturer specifications.
Temperature measurements become more useful when combined with airflow, static pressure and operating data.