CardBus PCMCIA Cards can be differentiated from the 'Plain Jane' cards by ridged strip (usually gold in colour) located near the 68-pin connector: This makes it so that a CardBus card can only be inserted into a Cardbus slot.

While, if you actually managed to get a CardBus Card into a JEIDA 2.1 slot, it probably wouldn't hurt the card, the card would not work anyway, because cardbus cards use different pins for voltage supply compared to regular PC Cards.