A simple and experimentally convenient method for determination of the twist elastic constants of the c-director in tilted smectic phases is proposed. The method consists in measurement of changes in electrical permittivity at high frequencies (about 1 MHz) caused by a DC electric field applied in the direction parallel to the smectic layers. A sensitive method for detection of changes in electric permittivity has been developed. The computer controlled apparatus performing the whole experiment is described. The results of measurement of an effective twist elastic constant in a ferroelectric liquid crystal are presented.