One trick to test whether a frying pan is hot enough is to sprinkle water on it. If the surface is sufficiently above the boiling point of water, droplets will skip across the pan. Those jittery beads of water are held up from the hot pan by a cushion of steam. The vapour cushion collapses as the surface falls below the 'Leidenfrost temperature', causing furious bubbling and spitting when the water droplet hits the surface and boils explosively.