Put a low value resistor in series with the earthy end and measure the AC volt drop across it. If the frequency is too high for your meter, use a diode probe to rectify the AC, there will be errors if the voltage is below, say 1.5V. If the indication on the meter, which is measuring DC V, is low, note the reading and feed the diode detector with 50 HZ AC to get the same DC output. Then measure the 50 HZ AC with your meter. You might have to change the time constants of the detector fpr the 50 HZ measurement.
Frank