To Determine the Meters Sensitivity, You Need a regulated power supply of a known voltage (12 Volts would be Good.) and a Few Potentiometers.
Start with Pot set to a High Resistance Pot. Possibly 50K
If No Reading on the meter as you get down to the last Quarter of Turn, Than remove it and try a 10K pot.
Set the Pot to its Highest Resistance and Connect it in Series with the Supply and Meter.
Than SLOWLY adjust the Pot till you get a Full Scale Reading.
Now Disconnect the Pot and Measure its Resistance.
Lastly, Divide the Volts by this Resistance to get the Meter Current.
Also, If you put Two Diodes, Back To Back, Across the Meter Terminals, It can give you some Protection against Damaging the meter.
Want to TALK?
If you have a LAND BASED PHONE, Finland is a FREE CALL for Me.