If you turn on your browser's developer tools and navigate to a site like Weather Underground (which extensively uses the V3 API), you can capture all the URLs it is calling. So far, Yincognito seems to have uncovered every one that they are using (other than incidental ones for serving ads and whatnot). It is interesting that WU makes occasional V1 and V2 calls, in addition to the main V3 ones.
FYI in case one isn't aware, Weather Underground was bought out by The Weather Channel a while ago, so WU and TWC are the same.