We propose a new technique for evaluating the RF voltage across the electrodes of low-pressure capacitively-coupled plasma reactors when direct measurements are not possible. It is based on determining the coordinates of the turning point in the RF breakdown curve and using known values of the electron drift velocity for the gas. The results are in good agreement with those obtained by direct measurements at the driven electrode. Furthermore it allows RF breakdown curves to be determined for different frequencies, giving results that are physically reasonable (coincidence of right-hand branches) and in agreement with other published results. The technique for determining RF voltage we proposed is valid when there is no discharge plasma between electrodes (e.g., before gas breakdown), as well as for negligibly small discharge currents (before extinction of the weak-current discharge mode).