In U.S.: Let’s be straight. Korean food is not yet at the place where, say, sushi is–that is to say, Korean food is not yet mainstream enough for one to expect a dependable taste for it, outside of the people who grew up eating it constantly. This necessarily means the ceiling for the quality of Korean food sold in areas sparsely populated by Koreans will be rather low. In the U.S., Koreans mostly live in Southern California, New York/North New Jersey and D.C./Maryland/Northern Virginia. (The next tier of Korean American population centers are Northern California, Atlanta, Chicago and Seattle, but the drop-off is significant after the top three.) The quality of Korean food tends to track that order.
Both in the U.S. and in Korea, this holds true: watch out for restaurants that sell too many different dishes, because every one of the dishes will be mediocre-to-bad. Unfortunately, this applies to vast majority of Korean restaurants in the U.S., because there simply is not enough demand to specialize in certain types of Korean food. In small towns of America, the menu often crosses the border to include Japanese/Chinese/Thai food, creating an unholy alliance of shittiness. Avoid this type of restaurant at all costs.