The spin-galvanic effect and the circular photogalvanic effect induced by terahertz radiation are applied to determine the relative strengths of Rashba and Dresselhaus band spin splitting in (001)-grown GaAs and InAs based two dimensional electron systems. We observed that shifting the delta-doping plane from one side of the quantum well to the other results in a change of sign of the photocurrent caused by Rashba spin splitting while the sign of the Dresselhaus term induced photocurrent remains. The measurements give the necessary feedback for technologists looking for structures with equal Rashba and Dresselhaus spin splittings or perfectly symmetric structures with zero Rashba constant.