Most merchants that also accept AMEX have a little sticker in their window. There are actually a lot more businesses that allow you to use an AMEX without a surcharge than there used to be: https://maps.americanexpress.com/aushopsmall#/

If you get time to watch the cycle of logos on the Paypass terminal while you're waiting, it will usually display whether it accepts Amex or not (and also logos now for Apple Pay & Android Pay too).