If the issue is not so much choosing a method as "characterizing" the work of a researcher into one category or the other, then there are any number of ways to do this -- e.g., the kinds of methods used, the kinds of questions asked, or epistemological assumptions. Whichever criteria you use, it will be difficult to make a hard and fast distinction, as certain kinds of work will always blur the boundary -- with mixed methods research being a good example of this.