All of the smart chargers that I have seen just look at the voltage that has been reached. Some look for a drop in the charge current as their intended voltage is approached. This is fine with a uniformly healthy pack, but not with a pack where one or more cells has got low capacity. The dud cell can come up to voltage very quickly leaving the rest to catch up when charging stops, usually in less time than seems reasonable. This is why I prefer "dumb" overnight trickle chargers. The dud cell gets re-formed, the relatively low charging current doesn't do the rest of the pack any harm.
A weak point on NiHM packs is the welded strips connecting the cells. When they start to corrode, they become a high resistance, reducing the current that can be delivered, and dropping voltage under load. Fortunately, a dead one can be cured with a big enough soldering iron and reasonably heavy wire.