if you just want to check the voltage just do the end that hooks up to whatever you are using it for. If you want to do it on the balance end then do this. Pretend the pins are numbered 1 2 3 4, measure pins 1 and 2, pins 2 and 3 and pins 3 and 4. All of them should be really close to each other for the pack to be in balance. Usually 0.02v or less. If they are more than 0.2v apart though then they are probably too out of balance to be using.