In the Middle Ages, there were many fake alicorns (fake unicorn horns) for sale, so they had to have a few things that would show you if the horse was real or fake. There is a list below.
Drawing a ring on the floor with the alicorn. A spider placed in such a ring would not be able to cross the line, and in fact would starve to death trapped within the circle.
Placing the horn in water, which would cause the water to bubble and appear boiling, even though the water would be cold.
Placing a piece of silk upon a burning coal, and then laying the horn on top of the silk. If the horn was real, the silk would not be burned.
Bringing the horn near a poisonous plant or animal, which would burst and die in reaction to it.
