I would put a 10K linear pot across a low voltage adjustable DC power supply, starting with a very low voltage (eg. 1 V). From the wiper, connect a 1K high precision resistor (1% or less) in series with the meter, and other terminal of meter to bottom of pot. Wind up the pot until the meter reaches full scale deflection. Wind up the supply if needed. Measure the voltage across the 1K resistor. The meter current is Im = Er / 1000. Measure the voltage across the meter, the meter resistance is Rm = Em / Im.
Make sure the meter is oriented in its intended operating position. If it will be in a steel case, or will have nearby iron, mount it in the case. Check the zero adjustment. These factors influence the full scale reading.