My thoughts on IDing a lone bird as to Greater or Lesser; If you "Think" you have a Greater then it most likely is a Lesser; Greater are "Big" birds. Greater Yellowlegs on average are slightly over twice as large as Lesser Yellowlegs by weight and their bills avg. 60% longer. Their bill length is usually 1-1/2 the head depth and the nare noticeably farther from the forehead than is seen on Lesser. The dark, heavy based bill usually has a slight up turn and is gray at its base.
The Greater's wings usually extend just past the tail where Lesser avg. a much longer wing extension and this is usually easy to see at a distance giving the daintier Lesser a more attenuated look while the Greater's short wings add to their blocky, heavy look. If a bird spreads its wings and you see spots on the secondaries the bird is a Greater as the Lesser's secondaries are plain. The Greater Yellowlegs is more closely related to the Greenshanks and Marsh Sandpiper than to the Lesser Yellowlegs.