> If you’ve ever used a debit card to purchase something in the U.S. you may have been asked “Debit or credit?” and wondered “Hmm, can’t they tell from looking at it that it is a debit card? Or isn’t it, I don’t know, in the card number or the magnetic stripe or somewhere?”

I don't know if it's a cultural difference, but at least here in Brazil, it's common to have the same physical card work as debit, credit, and as an ATM card. AFAIK, they are different applications running in the same chip on the card, selected by a command from the terminal. So they really have to ask "debit or credit", since the same card can be used for both.