If you're cooking with a batter, you can test whether or not the oil in the deep fryer is ready by dropping a bit of batter into the oil. If your deep fryer has a deep-frying thermometer that will indicate the temperature, you can stick it into the pot to get a reading. If not, wait a minute or two and then check if bubbles are forming around the element rod in the center of the heating element. When you start seeing them, the oil is hot enough, and you can then add food.