They use a thin film of resistive material with heat sensitive crystals on top. By pushing the two dots, you close the circuit, and the resistive material will warm up. Depending on the temperature, the heat sensitive crystals change color. The higher the temperature, the more crystals will change color, which in turn is an indication of the shape the battery is in. Personally I use a digital battery tester, it helps me compare different makes & types of batteries, under load.