If your charger has a separate cell voltage option and IR reader, check it on the charger first. If not you'll need a separate tester / checker.
Any ordinary tester / checker will work. You probably want one with a screen, and it checks voltage, total and each cell voltage, and IRs, total and for each cell.
If it's not charging all the way, that can be that the IRs are high / apart, cells are degrading / getting bad, cells are unbalanced, or it can simply be a setting in the charger.