(1) Look at least vaguely Asian. Sorry, I know this precludes a lot of people, but if you are Asian(-looking), then people will have a much higher chance of perceiving you as Korean; you just probably come from a far-away province with funny genetics.
(2) Have pale to lightly tan skin. If you are tan, forget about it. They can so tell you do not use BB cream religiously.
(3) Wear bold glasses. The bigger, the more hipstery, the better.
While in China, I have gotten a lot of questions about my ethnicity. I always am mistaken as Korean or perhaps Japanese. Here’s how the conversation goes:
Michelle: “Why did you think I’m Korean?”
Person: “It’s your glasses.”