Having a voltage graph of each pack definitely helps identifying the pack(s) havingtrouble. Assuming all packs started out with reasonably similar capacities.
Example#1, if a pack has difficulty reaching the same top voltage as other packs, despite the balancing function working overtime, then that pack has one or more cells with very high self discharge.
Example#2, if top balancing was working fine, but discharging towards ca 3.2Vonepack's voltage drops much faster than the others, then that pack as somehow lost capacity.
As to your actual question: Using my graphs,I've so far detected 8 bad cells. On 7 of them there were visual cues that something was wrong: "rust" from leaking electrolyte and/or abroken fuse.
In just one case, I could not see anything wrong. I plugged it back in and let it runfor a few days until the weekend, by which time "rust" had formed on the positive end of one cell.
When doing such maintenance, I also add some capacity to the weakest couple of packs by replacing one cell each (eg. 1700mAh -> 2300mAh). So it is conceivable that I have dead cells in my pack that have failed very gradually over time. I suppose I'll have to retest all cells eventually... perhaps in a couple of years time.