While it may seem daunting to be able to tell these two species apart, its actually not as bad as one may ponder. I’ve included two images below, one of each with links to each of their respective photo galleries offering additional images.
The easier ID points to look for are the bill and body size. The Lesser Yellowlegs has a noticeably shorter and straight bill whereas the Greater Yellowlegs almost always has a slightly upturned and longer bill. In addition, their body sizes are different whereas the Lesser Yellowlegs is smaller. If you are familiar with a Willet, the Greater Yellowlegs is roughly the same size. Another ID point is the more extensive bars on the Greater Yellowlegs flanks. And of final note, the Greater Yellowlegs has a harsh call vs. the Lesser Yellowlegs which has a mellower tone. In Orange County, our records show there has been slightly more sightings of a Greater Yellowlegs then a Lesser however both can sometimes be seen together which makes separation quite a bit easier. Both can reliably be found year round.
A video of the Lesser Yellowlegs can be seen below these images.
I’ve uploaded a video for the Lesser Yellowlegs recorded on February 2, 2016 at the Santa Ana River