Use a cooking thermometer to check for the readiness of the oil before deep frying. It should be between 350 °F (177 °C) and 375 °F (191 °C). If you don't have a cooking thermometer, test with a wooden chopstick or spoon. Dip it into the pre-heated oil. If the oil starts to bubble steadily around the chopstick/spoon, it is hot enough for frying. If there is very few bubbles, it is not hot enough.