How it works: Alcohol vaporizes at a much lower temperature than water so if your pot starts to boil, it's too hot.
If the temperature is low enough, all that will vaporize is the alcohol. The alcohol vapor will rise to the top of the pot. Because the wok or bowl is filled with ice, it will cool the vapor and cause it to condense on the bottom side of it. Gravity will cause the condensation to run down the wok or bowl until it reaches the lowest point in the center. At that point the condensed alcohol will drip into the glass.