The only non intrusive way I can think of would be to have a sensitive thermometer and run it along the pack when charging - the dud, depending on the mode of failure, would show a different temperature.
The other way is to use a pin as a meter probe, and poke through the insulation of the pack, noting the voltages cell by cell. Not really recommended, since you wind up with a pack with a row of pinholes in the insulation. If the application of the pin is over enthusiastic, there is also the risk of puncturing the cell being looked at, creating a further problem. This only applies to NiCads and NiMH batteries, I wouldn't consider poking at Lithium, and since SLAs are effectively a sealed unit with no consumer serviceable parts.
Of course, when the dud has been found, the pack is in for either surgery or burial.