Sue,
They are pretty rare in the US and I believe they are all marked and advertised to their users. Not all of them are shaped like traditional credit cards such as the Mobil SpeedPass.

I think Visa offers one called the "Blink" and American Express also offers one. The Visa has a special symbol on it and the Amex has some embeded-electronic looking stuff on the front of the card. Use Bing's image search for RFID Credit Card to see examples.