Comparing a Least Sandpiper and a Western Sandpiper to the untrained eye can be a bit of a challenge especially in basic/winter plumage and if solo. In this photo, taken on 1-28-2016 at Irvine Lake in Irvine, California we can see both side by side.
The Least Sandpiper, typically shows its yellow legs that can make IDing this species far more simple. However a leg ID is not always available as in this image where the mud obfuscates its color. Our next analization point might be the size. The Least (on the left) is noticeably smaller. Also it’s bill is considerably shorter and as a 3rd ID point, its chest shows more extensive light brown plumage to its upper belly. The Western Sandpiper in contrast shows a larger bill, noticeably more white on its chest and belly and the Western Sandpiper is larger in size.
Hope this helps some of you out there facing this ID dilemma between the two.