The part to test is the hypothalamus and the preoptic area
Information on the surrounding temperature is brought from the periphery by nerve afferents from cutaneous thermoreceptors (sensitive to changes in skin temperature). But the most important element in the perception of thermal information seems to be the fact that there are heat-sensitive or cold-sensitive neurons in the brain. Very high densities of thermosensitive neurons are found in the anterior hypothalamus and the preoptic area.
There are two types of heat-sensitive neurons, those that are sensitive to heat ("hot" neurons, which considerably increase their activity [electrical discharges] when they are warmed up and decreased when they are cooled) and those that are sensitive to cold ( "cold" neurons that do the opposite).