i suppose you talk about the body diode of the mosfet, right? If so, it will indeed have a voltage drop which will be visible when current flows, but it is typical less than normal diodes, about 0.4V. Depending on the type of the fet, it can be as low as 0.2V at low currents. So you wanna add a resistor to the output to have some current and then measure across the fet at the scale of mV