The terminals in Portugal definitely show a DCC prompt but the implementation is very tricky. The experience feels designed by the merchant banks to get you to inadvertently go for DCC. I would ALWAYS have them place the reader down on the table in front of you before tapping and keeping it until a receipt has been printed.

The implementation in Portugal (both at merchants and at ATMs) seems to be:

1. Ask if you want to be charged in USD (1) or Euros (2).
2. It then asks again using completely different verbiage: ACCEPT CONVERSION (1) OR REJECT CONVERSION (2).

The 2nd prompt is what may catch you out, because if you don't read it properly, you might assume it's asking you to continue the transaction, but it's NOT, it's still the DCC. Also, if you hand the terminal back to the merchant after just the first prompt, then they can then still (inadvertently or purposely) DCC you.

My experience with this has been that tourist spots have "mistakenly" DCCed me more than others, which make it seem less like a mistake as one would assume they're more familiar with foreign cards than the local establishments where it doesn't happen. In one scenario, a merchant literally saw me hit EUR and I told him EUR please and then he grabbed the machine before the second prompt and out popped a receipt with DCC on it.