Least Sandpiper is on the left & Western Sandpiper on right.  First tip, color of the legs.  Least is yellow (greenish) and Western is black.  Often is hard to tell in poor light, distance, or mud on Least’s legs.  Least is a warmer brown than the Western’s gray tone on back.  Western has a larger white patch in the shoulder.  You can’t see in this photo, but the Least has a very fine-tipped, slightly decurved bill and the Western has a much longer, more decurved bill.