As I said in the post above, I think once the DCC verbiage prints in places like China, it's too late. The way it works in Taiwan, Thailand, and a few other places is that you'll get a quote slip with the DCC verbiage and tick next to the local currency (DCC opt-out) or card's issued currency (DCC opt-in). The cashier will then print out a final receipt showing the [X] on your selected choice. The DCC verbiage still prints on this final receipt, but at least you have some proof of your choice being respected.

In the case where you check the box but don't get a followup receipt showing the [X] on the final customer copy you really have no way of knowing whether the cashier input your currency choice properly. From the stories on this thread and others on the topic, almost universally this kind of system results in DCC.