To know when the fat is hot enough to saute foods, wait for the milky white foam to subside. This means that the water in the butter has bubbled off and evaporated at 212ºF (100ºC), allowing the butterfat to heat up. This will reach around 350ºF (177ºC) for lightly browning the food. Using temperatures any higher, I recommended using clarified butter to ensure the milk solids don't burn in the hot pan.