NiMh charging stops when the charger registers a delta-peak on the pack voltage. The charger will supply constant current as long as the battery voltage continues to climb, once it drops off from it's peak by the threshold amount programmed in the charger, it will stop.

Some chargers have you set how many cells are in the pack you're charging so it can register a fault if the voltage is very different than expected, but as long as the charge rate is 1C or less, you shouldn't have any problems.