All you need is a DC VoltMeter. Place the red electrode on + and the black on - and if you have the voltage set properly in DC, it will say something like 1.3 (which would be remaining volts, for a 1.4V or 1.5V battery).
There is a certain percentage at which time you throw away the battery, I believe when it has lost 30% of its rated power (a 1.5V DC battery should have at least 1.05V remaining at 30% loss – a 1.5VDC battery should have at least 1.2VDC remaining for a 20% loss).
I may be out in left field with this as it has been a long time since I have calculated for electricity and electonics.
Feel free to correct any mistakes in the above. 
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