where is the PIN on a gift card

i want to check the balance of gift cards that i gave as a gift to other family members. I'm trying to see how effective the gift was -- did they use they use the balance or not. (it goes without saying that i don't want to ask them this directly.) i was at the point to check on it when the screen asks for the PIN. i presumed it was the 16 digit number on the back of the card. when i enter that, it says that is not a valid PIN.