From the Internet - temp quoted in Farenheit.
To do the delta temperature (dT) test, simply insert a thermometer into
the inlet and outlet of the radiator. If using a digital pyrometer,
the thermocouple can be slid between the hose and the spigot and the
pipe clamp retightened to seal it in. Then just drive the car
under whatever conditions are giving you trouble and observe the
readings. My dual input pyrometer will display the difference between
the readings directly.
What you are interested in is the temperature loss across the radiator
and the actual temperature of the outlet. The radiator should drop
at least 20 degrees and preferably more. More importantly, the
outlet temperature of the water should be below the setpoint of your
thermostat. If you're running a 180 degree thermostat, an outlet
temperature of 165-170 should be observed even in the hottest weather.
If the dT is low and/or the outlet temperature is high, that indicates
insufficient heat transfer. That could be either low airflow or blocked
tubes. If the dT is OK, the outlet is lower than the thermostat setpoint
but the INLET temperature is significantly higher than the thermostat
setpoint, that indicates that there is insufficient coolant flow.
