For optimal safety, you should use a frying thermometer at all times, but for those occasions when you do not have a thermometer, you can use this old-fashioned method for approximating the temperature of the frying oil. Note that this only approximates the oil temperature.
Heat the oil over medium heat. Avoid using high heat, which could cause the oil to heat too fast and ignite. See the warning.
Drop the bread into the oil and watch both the bread and the time on the watch.
Watch for the bread to brown on all sides and float to the top to know when to begin frying your food.
Watch the time for browning of the bread: 20 seconds means the oil's temperature is between 194 and 199 degrees Celsius; 40 seconds to brown the bread indicates 365 to 382 degrees and 60 seconds for the oil to brown the bread shows the oil at 350 to 365 degrees.
Do not expect to get exact results without a thermometer, but use the range to determine when to add your food.
For best results when deep frying, use any of these high smoke point oils: peanut oil, safflower oil, refined sunflower oil and soy oil.
Always have a fire extinguisher handy.