You can learn a lot if you take a look at
www.rcbatteryclinic.com/
With either NiCad or NiMh you can just put them back on the charger, no matter what the current state of charge is. If you are using the wall wart (the charger that is usually packaged with a transmitter) you can just charge all night before you go out to fly. At the low rates that the wall warts put out, no harm will be done if you do a few hours of overcharge and you will be assured of a full charge for the start of the day. If using a smart charger and you set it for the proper battery type (NiMh and NiCad have slightly different peak voltage drops which is what the typical smart charger uses to terminate a charge) you can charge a partially discharged battery as well as a fully discharged one. The charger will detect when the battery is full. Do not forget, you will benefit from looking at Red's site referenced above.