How closed loop temperate control works in Ecigarette / Vaporisers
It's "ohmic", so if you know the voltage across it and the current flowing through it, you know its resistance.

Because the change is quite small, you need to be careful with the measurement. You probably should use four-wire "Kelvin" measurement for the voltage across the element, and to measure the current you need another resistor of a known, small value called a shunt. And you need to make sure that its temperature does not affect the reading too much.