You can used mutimeter to checked your battry. Used red point to positive and black point to negative to checked your battery and then you can checked the batter amp. How to checkthe amp. Used positive to your motor red and back connect to negative.The batter nagative to motor nagative point.
I might not quite be able to catch what did that CHing mean. Anyway, I think what he meant was about connecting an ammeter in series with a brushed motor powered directly by a pack of battery. The ammeter will show how much current the motor is drawing but it can't tell whether or not the battery is dead. To know the condition of a battery you definitely cannot tell only with a multimeter. You firstly need to have the cells' datasheet on the discharge characteristics and conduct charging and discharging tests. The result is not a go or no-go answer unless the cell is really completely dead. You can only know how much the cells have deteriorated due to aging or misuse. For hobby purpose, I would say if the capacity has deteriorated by more than 20%, you can consider to throw it away.