A merchant cannot do DCC without the supporting fields on their POS system and processing interface. Thus nearly all ATM networks, whether ones owned by processors, MC or V, or bank-owned ones, do support DCC. the American Express processing infrastructure has no DCC capability so it is not possible to do DCC on an American Express transaction.

However, (there's always however), in some countries American Express does not handle themselves but are processed by a local entity (e.g. in Brazil there is no American Express network as such, it is Bradesco). In principle it still should be impossible to have a DCC on American Express-branded plastic in that situation, but the capability does exist in the multi-card processing systems even though it is required to be not enabled per American Express agreements. Is it possible anyway? Maybe, although I have never heard of it happening. Of course DCC is far from the only way to fleece unsuspecting consumers.