It is important to control the skin temperatures of smartphones, tablets, and wearable electronics. The comfort levels of the skin temperatures are determined by surveys with subjective evaluations on a limited number of configurations and/or materials. This study is the first attempt to develop a characterization tool that is objective, repeatable, and predictable. The first step is to apply a gel finger that replaces the human finger to measure temperatures after gel finger's contact with the skin, i.e., the case of a mobile system. The second step is to establish a model calibrated by the experimental results. The calibrated model can be used to simulate different effects on the temperatures at the interface between the gel finger and the skin. The temperatures of the polycarbonate skin “felt” by the gel finger are always lower than those of the aluminum skin. The difference could reach 2–6 °C depending on heat spreading inside the system and heat sources in the finger. The difference can be reduced from 6 to 3 °C by using a novel casing with thin film metal on polymer. The transient periods are approximately 100–200 s with shorter transient periods for the aluminum skin.