Through the balance plug. A balance plug allows you to individually check the voltage of each cell in a pack. This allows you to balance charge a pack and match the voltage of each cell.

If a pack doesn't have a balance plug it normally has a protection circuit installed for a similar task. Hopefully it will account for errors in judgement. I don't have a hand on many consumer cells but if a battery takes 8 hours to charge at normal charging current I would be extremely suspect of that battery bms or no.