A new diagnostic method is proposed for direct measurementsof electromagnetic field patterns of fast waves in toroidal plasmas.The fluctuations of the ponderomotive potential at the beat wave frequencybetween two fast waves with different frequencies produces the actual potentialfluctuations via electron and ion motions. A heavy ion beam probedetected the potential fluctuation patterns at the beat wavefrequency during the fast wave pulse. The amplitude of the potentialfluctuations decreases with increasing electron temperature in fastwave current drive experiments in JFT-2M, owing to theimprovement of wave absorption via electron Landau damping. Themeasured potential fluctuations do not contradict the predictionusing full wave code calculations.