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Any pictures or part numbers, any markings at all? Might try the manufacturer's website and see if the sell replacements, or maybe any useful information about problems with their product. Might unintentionally spill some of their secrets...
I don't know for sure, but both seem to have similar charging charactoristics and capacities.
Charge them up to full power, then just leave them sitting there in a warm place...if their votlage drops considerably quickly (I forget the numbers...5% per day?) then it's NiMH.
What are you charging them with exactly? If it's a good computerized charger then you probably should know what type they are. If you are using a NiCd charger to charge them and they aren't getting damaged, then it's NiCd. If they are getting damaged, they are probably NiMH. if you are using an NiMH charger, then there is no way to tell (NiMH chargers work with both types).
Green usually indicates NiMH, but that's hardly conclusive evidence...
Over at the Panasonic website, their NiMH handbook indicates that both batteries will lose ~20% of their charge at 45C over a period of a week, with the NiCd being worse by a few percent.
The only other difference (other than capacity) is that the NiMH discharge curve is more sloped than the NiCd - NiCd will quicky drop to 1.25V within 1.5hr and stay there when discharged at 1/5C for the next 3.5hr. The NiMH will quickly drop below 1.3V within an hour, then gradually drop below 1.2V after 3hr.