If your charger reads IR's (internal resistance) then you just keep an eye on those every time you charge your batteries. If you start to see the IR's climb from charge to charge, you know you have a cell or cells going bad.
You could throw the LiPo's in a pool, and they will sink to the bottom and melt through your pool liner. They create their own oxygen. So they would have just kept on burning had you even had a brand new extinguisher and emptied it on them. Now, that would be fine if the LiPo's were done...