In winter, the Western Sandpiper (Calidris mauri) can easily be confused with the Semipalmated Sandpiper (Calidris pusilla) and the Sanderling (Calidris alba). 
It can distinguished from the Semipalmated Sandpiper by its relatively long sharp-pointed bill which is drooping slightly at the tip.
In winter, the Sanderling (usually) has a black patch in the shoulder which lacks with Western Sandpiper. 
Sanderling (Calidris alba), Semipalmated Sandpipers (Calidris pusilla), Western Sandpipers (Calidris mauri) and Least Sandpipers (Calidris minutilla) all occur in the US and can easily be confused with each other, especially in winter.
Least Sandpiper, however, has greenish-yellow legs while the others have black legs.
A recently (2013) discovered reliable field mark is that Sanderling (and Least Sandpiper) has a distinguised gape notch ("hook" or "line" at the base of the bill), in all plumages.
Semipalmated (Calidris pusilla) and Western Sandpipers (Calidris mauri) don’t really show that gape notch. 
Most field guides don't mention this feature.

1st & 2nd photo: In the company of the slightly bigger Dunlins. Galveston, Texas, USA, 8 April 2007
3rd photo: In the company of a Piping Plover. Bolivar Peninsula, Texas, USA, 28 October 2006