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).