To test battery health and determine if a cell may not be performing you may perform a voltage check on all batteries in the string when they beginning the discharge, i.e. just after they were brought to a full charge.  If a battery contains a failed cell, you will notice a difference in the voltage under load when comparing voltage readings from one battery to the other.

More detail on this type of test can be found here:

https://www.youtube.com/watch?v=u5vIQIuFaME&t=721s