Measuring Load Capacitance and resistance of 4~20mA transmitter
A loop powered device should appear as an extremely high impedance (a current sink) over the compliance range, so you can simply change the voltage and measure the change in current. It would be reasonable to use mid-scale (12.00mA) if you're looking for a typical measurement. So you could vary the voltage from (say) 10V to 30V and calculate the resistance from Req = \$ \frac{\Delta V}{\Delta I}\$. If the current changes by 1\$\mu\$A (say from 12.000mA to 12.001mA) for a +20V change, the equivalent resistance is 20M\$\Omega\$. Usually you might pick voltages that are not quite at the extremes of the guaranteed compliance range in order to get a good-looking number for the datasheet (and to more accurately represent typical operating conditions). For example, if the device will work from 8V to 60V you might use 10V and 48V. 

The output resistance is gain-dependent (on the internal circuitry) so it will likely vary quite a bit from unit-to-unit, as well as over temperature (it's not unusual to see a 5:1 or 10:1 ratio between typical precision op-amp gain at 25°C and worst-case gain over temperature and unit-to-unit variations) so be careful about quoting a guaranteed spec based on a single measurement at room temperature. 

Capacitance of a loop powered device will tend to be the shunt capacitance in the schematic at high frequencies (usually there's a 10n or something like that effectively across the loop terminals).