If you already have a credit card with Stripe then definitely yes, you know the merchant doesn't have your card number. IIRC, when you first enter a credit card you remain on a web page controlled by the merchant, so it would be hard for Stripe to guarantee the merchant didn't intercept it in any way. Since this guarantee isn't possible, they're not going to show a message that says the credit card is stored at Stripe and not the merchant. It's up to the merchant to show such a message.