To find the broken light on a Christmas tree, start by looking for any flickering or dimming lights. If you notice a light that's not working properly, check the bulb to see if it's broken or missing. If the bulb is fine, check the wiring to make sure it's not loose or damaged. If the wiring is ok, try swapping the bulb with another one to see if that solves the problem. Finally, if you still can't find the problem, try turning the tree off and on again.