Anything and everything: Official readings, banks or other pubicly visible readings while driving, other cars, other thermometers close by. It's almost always higher. In the winter it's not so bad.Against what other temperature source are you comparing it against?
I get that bank signs might be inaccurate, but when the official high for the day was 90 degrees where I live and my car was saying 98 or even 100 degrees even after driving for a while and this happens all the time, I know it's off.All those sources you cite are notoriously inaccurate. Those public signs, like at banks, are the worst.
My suggestion is that your car is more accurate than you think. It can easily be validated by the use of an infrared thermometer.