"Is there a standard test to determine if the oil is hot enough?"

A thermometer that clips to the side of the pot is what you want. OR a reliable electric frying pan that you can set to the required temp. The magic number is 375F. Only add two or three doughnuts to the hot oil at a time ... too many will cool it down and you'll get greasy squirrel doughnuts*.

Frying doughnuts can be a bit tricky... by the time the center is cooked, the outside is overcooked, thus the reason for donut holes. Besides, it's been scientifically proven that there are no calories in doughnut holes, no matter how much powdered sugar you put on them.

* Always cook squirrels separately.

Sue