Hi Turbo Scorpion, you can measure individual cells in a stick by piercing the insulation on side of the cell you want to measure (its -ve can) and the adjacent cell located at the +ve end, simples!
Back in my old 1/12 days we used to skim a patch off the cell sleeving to allow us to measure each cell when charging and check for imbalance. With multi cell lipos this is now standard procedure with high tech balance chargers of course!
I have also seen dodgydiy's response and he is quite correct too but using a 12 volt you do have to be careful as he suggests, the cell may heat rapidly and vent its electrolyte or explode of course, hence hiding behind a wall!
I have done this procedure a few times in the past, drop the cell on the floor first and usually using a charged 7.2 volt pack to kick an over discharged cell back to life.
You will never recover the full cell capacity though and it will be the weakest in the pack with a risk off reversing polarity under heavy pack discharge!
Happy bomb making, seriously do be careful if using high voltages. John