When you get the slip to sign, the slip is supposed to show 2 currencies, you're supposed to ticket the currency, AND they're supposed to give you another slip to show the currency that you ticked earlier, then you sign that.
There's always a DCC verbage at the bottom of the slip, remember to read that before you decide to sign or not. If you see it, tell them you don't want DCC and want to be billed in their local currency. The problem with this is that sometimes they person may not understand and sometimes the merchant's bank put a lock on the terminal to force DCC (hence the person can't cancel). So there are genuine cases where it isn't the person's fault, it's more of the bank. This happens frequently in China (I've see it in KL as well).
I use an Amex card to avoid all these issues since DCC only affects Visa and Mastercard. Sure, it's more expensive to use Amex but still cheaper than getting DCCed.
If your bank statement doesn't show the foreign amount next to the transaction, that means it is DCCed.