How can you tell if the milk is scalded without using a thermometer?

When milk is heated, the proteins unfold and begin to agglomerate, forming a thin layer on the surface of the milk. As the temperature increases, the proteins continue to unfold and the layer of agglomerated proteins begins to thicken and break up into small fragments. These fragments then rise to the surface of the milk and form a wrinkled, leathery-like layer known as a 'skin'.

Scalded milk will also have a distinct cooked flavor.