How can I determine if a cell is working correctly i.e. if the cell heart has to be replaced?

The easiest indication for a damaged cell is the voltage: If a single cell needs >15V for sustaining a current of 200 mA then it should be replaced. A cell voltage below 3V (at 200 mA) is an indicator for a short-circuit within the cell, which leads to zero production.

The most important indicator is the ozone production rate. This can be verified with a check measurement. It is possible that the cell voltage and current are within normal parameter range but the cell does not produce the expected ozone quantity (e.g. damaged cellheart). If the ozone rate is just too low the lifetime of the cell can be extended for a short period by increasing the current to 300 mA.