An example system includes a patch defining a first surface and a second surface opposite the first surface. The first surface is removably attachable to skin of a subject. The patch includes a conductor associated with the first surface, a first sensor configured to determine a temperature of the conductor, and a second sensor configured to determine an additional temperature. The first sensor is separated from the second sensor by material having a known thermal resistance. The patch also includes a transmitter operably connected to the first and second sensors.