Abstract: A method for identifying overheated spots in liquid-filled electrical devices, comprising the steps of: a) determining the locations of potentially overheatable spots in said device and mapping said locations; b) positioning a tag consisting of one or more chemical indicators on potentially overheatable spots in said devices, wherein when said tags are exposed to a given high temperature, they are depolymerized into thermal degradation products which are diffused into the liquid; c) identifying the thermal degradation products by analytical methods; and d) locating the overheated places according to the identified thermal degradation products and the map of locations of said tags; wherein the tags comprise polymers and copolymers, which are substantially absent from the liquid of the device at normal working conditions.