Apple juice begins to boil when it reaches a temperature of 160 degrees Celsius, or 320 degrees Fahrenheit. It reaches its boiling point at a higher temperature than water due to its sugar content.
Check to see that the apple juice has reached the proper 160 degrees Celsius by using a food-safe thermometer to measure its temperature. If a food-safe thermometer is not available, one can tell that the apple juice is boiling by inspecting the liquid. The juice is boiling after it has moved on from the bubbling stage, and it should be rolling and steaming within the pot.