Highly disordered superconductors, in the magnetic-field-driven insulating state, can show discontinuous current-voltage characteristics. Electron overheating has been shown to give a consistent description of this behavior, but there are other possible explanations, including an electric-field-induced breakdown of the insulating state and a novel "superinsulating" state. We present ac-dc crossed measurements, in which the application of a dc voltage is applied along our sample, while a small ac voltage is applied in the transverse direction. We varied the dc voltage and observed a simultaneous discontinuity in both ac and dc currents. We show that the inferred electron temperature in the transverse measurement matches that in the longitudinal one, strongly supporting electron overheating as the source of observed current-voltage characteristics. Our measurement technique may be applicable as a method of probing electron overheating in various other physical systems, which show discontinuous or nonlinear current-voltage characteristics.