Quite frankly, I wouldn't count on it. Sorry. Spain is still... not up to this-day-level-of-technology.
ETA: I wouldn't even count too much on them accepting a REGULAR Amex card, except for the expensive stores/ restaurants/ hotels at Madrid and the like. But for the regular places we mortals (aka the-not-rich-and-famous) frequent, I would take the "normal" Visa, Mastercard or even just plain cash.