This is a Korean vocabulary test and will provide you with an estimate of how many words you know based on a sample test of 100 words. I think this is the most accurate test available on the internet as of the publish date. Try it yourself to see.
This is the first of two tests. You may wish to do the second test as well to improve the accuracy of your result, or you may come back to the second test later in time to reassess your level. [Link to second test]