Remember, the probe measures "ambient" temperature, not "actual". Otherwise, you would get all sorts of variable readings depending on your speed (i.e. wind velocity affecting the readings).
The temperature readings at initial startup will not be correct. It stabilizes after about 5 minutes of driving.
So, I'm not sure if a simple thermometer test will do the trick...
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car