In the US, debit cards and credit cards are processed by the same systems (by the same few handful of companies), so usually the only way you can tell the two apart visually is if the debit card literally says "debit" on it.

Merchants have the option of verifying the cardholder by comparing the name to some photo ID but it's very uncommon. There is a box on the back to sign the card but in my experience, the Post Office is the only entity that has ever checked it.