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 .