How can I test battery for true mAh rating?
To really test a battery capacity requires a timed test with a constant current load on the battery. Since the battery voltage changes as the battery gets depleted trying to use a fixed resistor type load leads to the load current getting smaller as the battery voltage goes down. The constant current load varies its effective resistance dynamically to so as to keep the current the same over time.

For low current levels it is relatively easy to build your own constant current load device using an opamp, power MosFET, a few resistors and a capacitor or two. Power the constant current circuit from an external power supply. There are plenty of circuits on the web to go by.