Local electric field strengths in low density plasmas can be measured directly by Doppler-free two-photon spectroscopy of atomic hydrogen mostly present in such discharges. This method is based on the Stark-splitting of the atomic resonance lines and causes no significant perturbation to the discharge. For this purpose, we take advantage of our advanced pulsed UV-laser spectrometers which provide not only the peak power needed for two-photon excitation but also the high spectral resolution to resolve the atomic hyperfine splitting. In a first experiment with opto-galvanic detection, atomic hydrogen was produced by thermal dissociation in a small cell filled with hydrogen gas and the Stark-splitting of the 1S-2S and the 1S-3S/D transition was measured. Electric fields as low as 200 V/cm and 30 V/cm respectively could be determined in accordance with theory. In addition, we have performed measurements in a hollow cathode discharge which provides higher electric fields in its cathode fall region and the 1S-2S spectrum was detected spatially resolved by means of opto-galvanic and polarization spectroscopy as well. Selected experimental results will be presented which clearly demonstrate the high potential of this optical method.