You can always go the traditional route and use a wooden chopstick to check the oil - it's hot enough when bubbles form all around the object. However, it takes a bit of practice to know just when the oil reaches the right temperature. Add to that the fact that you need to adjust for a drop in temperature when placing the food in the wok, and the novice is better off relying on a deep-fry thermometer. If you don't have a deep-fry thermometer a candy thermometer will work also.