Well when you charge them the charge current goes down at the end as voltage potential gets smaller, best way if you don't have anything else is to make a discharge test, make a constant current discharge of 1A, when voltage gets lower then 3.2V (to be safe) note the time, if you hit 1A spot on then you have 3000mAh if it takes 3 hours for the voltage to go from 4.2V to 3-3.2V.