Read through this wiki, it will give you a good overview on the basics of the battery construction and what everything is called and how it’s supposed to work:
Then when you get the battery in your hands you’ll need to get access to the balance connector for the bms and a volt meter. You can get a cheap meter online or at harbor freight or similar hardware store. You don’t need anything fancy just something ok. Then test the total battery voltage as well as each p group voltage and record all the results. Test carefully not to circuit the balance leads or main battery output:
Your connector will look similar and you want to test across the wires via the clips on the back of the jst
Just as a place to start and not getting into the details here but every group (assuming it is a LiIon pack) needs to be at least 2.5v and no more than 4.2v. They should also be within a few tenths of a volt of each other - optimally within .03v. The more out of balance the more carefully it should be investigated. if any group is at less than 2.5v it will likely be a loss if the reading isn’t due to open connections and is accurate to the cell groups voltage, when sitting below the minimum voltage parts of the cell dissolve into the electrolyte and when it is charged back up it can make little short circuits that will put the cell in thermal runaway (fire bad) (@Battery_Mooch did i get that right?)