Best method I find with "smart" chargers, since the exact algorithms in use are unknown, is to actually charge a battery with it, while closely monitoring the charge progress with both an ammeter and voltmeter, and keeping an eye on battery temperature.
Of course, do make sure the battery being charged is adequately sized for the charger. Attempting to charge an undersized battery at a high charging current can result in a thermal runaway situation. Refer to the battery manufacturer's datasheet for the recommendation on maximum charging current.
Also, I've looked at the specs for your charger, and it seems it can be used as either a power supply -OR- a charger, not both simultaneously. Be sure you don't have any loads connected to the batteries during charging. Loads being powered from the batteries during charging can cause the charger to never go to float charge mode!
Of course, even with a properly functioning charger, being used properly, with adequately sized batteries, other conditions can cause a thermal runaway. The age and condition of the batteries can be a factor, as can high ambient temperatures (especially if the charger does not monitor temperature and adjust charging voltage accordingly).