Older lithium batteries often suffer from 'self discharge'. They discharge even when not used.
One way you could compare your batteries is to label them, charge them the same way. Then measure and record the initial voltage. Wait a while, maybe a day or three, and record how much the voltage has decreased. If you see one that drops more quickly, it is likely a bad cell. One gotcha is you will see a high voltage right after charging that will quickly drop. So probably don't put too much emphasis on that initial number unless it is particularly low.
On really bad cells, you might be able to just wait a week and see if the jb3 reports fewer battery status bars.
Another option would be to use each charged battery in exactly the same way and measure the voltage, or measure the number of battery status bars. So maybe that would be recording 30 minutes of the same content, or longer.
When measuring, don't short the batteries. They could explode.