Theoretically, awakening the nerd in me, channeling high-school chemistry here, since ethyl alcohol has a much lower boiling point than water, if you were to thoroughly incorporate the vanilla extract into another liquid ingredient that is tolerant to heating, and then heat the liquid ingredient to a boil, what will happen:
#1. The mixture will heat up to the boiling temperature of the alcohol, and remain there until the alcohol has boiled off.
#2. The mixture will then begin heating again until it reaches the boiling temperature of water.
Because we're talking a small total amount of alcohol, the boiling plateau of the alcohol will be relatively short, but if you can spot the two different boiling plateaus happening, you can know with 100% certainty that the alcohol will be gone when the second boiling plateau is reached. You can cheat by using a thermometer.