How to measure the surface temperature of my cast iron griddle?
Water and canola oil.
It is generally ready to go for most uses when water sprinkled on the pan dances and skitters. If the water just boils away, the pan isn't hot enough, and everything will stick like crazy.
The pan is just right for searing when the oil smokes. You could run the pan hotter, but you will just make lots of smoke and trash the seasoning.