I don't know if the trip current is measurable, but the tools that the electricians use is a simple resistor network connected between Live and Earth via a rotary switch. Using Ohms law, the resistor value's can be determined, ranging from as little as 5mA all the way up to 30mA (or more if required).
Most Earth Leakage (EL) breakers trip out at about 20mA, (or at least the ones in South Africa). The unit is housed in a little black box, and the three pin plug is plugged into any outlet socket. Starting from the lowest mA position, slowly rotate the switch (i.e. incrementing the mA leakage) and wait fot the EL to trip. Look at the mA setting on the rotary switch to determine the mA trip current.