there's no way to know what physical damage exists inside cells without physically destroying them to open them up and examine them (usually by some form of microscopy).
if you had access to individual disconnected cells you could run internal resistance testing to compare to spec sheet values for the cells, under load and without load, but this requires disassembly of the pack for that access, and that risks damage / shorts / etc if not done carefully, and reassembly of the pack afterward requires tools, parts, and knowledge that may cost more than the pack itself is worth, especially if it's older.
by "charged...below zero", do you mean that the battery was discharged down below 0v, so it is now a negative voltage? If so, that's probably damaged in a way that i myself would not want to risk re-use.
if instead you mean that you charged it below 0 degrees (c? f? etc?) then the results may be less predictable, but more details are needed. was the pack actually completely cold-soaked, so that it was internally at that temperature? or was that just the ambient temperature, and the pack itself was normal in-use temperature?