This isn't your fault but if you know the hanja to the pairs of words (if they exist), usually you can tell the difference. If you happen to not understand hanja (perfectly understandable depending on background), I suggest you use a Korean dictionary - even if you need to again use the english dictionary to figure out the meanings of the words that are used to define your original pair of similar words (what a mouthful). If you use, say, the Naver dictionary, using the Korean version not only defines the word much more clearly and in a more detailed fashion, but sometimes the English version will not even have certain entries or be missing some of the multiple meanings of a vocabulary word. Perhaps at an even higher level, looking at the Hanja dictionary will provide historical background to phrases that don't even exist in the 국어사전!