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.
My impression was that if you sign it, they’re not supposed to ask for photo ID because the card company is bearing the risk (they could instead check the signature against the one on your receipt)
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.