A new simple differential technique using two Langmuir probes yields an analog signal proportional to the local electron temperature fluctuations in a plasma, even in the presence of density and potential fluctuations. By externally driving a periodic change in the plasma temperature, it has been possible to check this technique and its accuracy and to enable calibration. This differential technique is used to measure the amplitude fluctuations in a turbulent collisionless drift wave spectrum present in a laboratory plasma.