Is it through scholarly impact/citation studies (like this one)? Or reputational surveys (such as here or here)? Or SSRN downloads (e.g., here)? Or is there no reliable way to gauge this? Take the poll.
UPDATE: A comment from a reader leads me to realize that I should emphasize that you should please interpret the categories broadly: so, e.g., "scholarly impact/citation studies" doesn't commit one to my way of doing them--it could include impact/citation studies that used different databases, that included books, Google scholar, etc. Similarly, a reputational survey needn't mean the way US News does them, but could include a survey in which experts are asked to evaluate recent work by colleagues and give their assessment of it.