The best way to tell is by using a frying or candy thermometer. If you don't have one, do it the way my Grandma (and I did when I lost my thermometer) did it. Place the end of a wooden spoon into your hot oil. If bubbles form around it and float to the top, it's ready. If it bubbles vigorously, turn down the temperature of the stove and try again in 10 minutes. Another way is to drop a piece of popcorn into the oil. When it reaches a good frying temperature, the kernel will pop! Fish it out and start making donuts!