Semipalmated Sandpipers (Calidris pusilla), Western Sandpipers (Calidris mauri), Sanderlings (Calidris alba) and Least Sandpipers (Calidris minutilla) all occur in the US and can easily be confused with each other, especially in winter.
The Western Sandpiper can distinguished from the Semipalmated Sandpiper by its relatively long sharp-pointed bill which is drooping slightly at the tip.
The Least Sandpiper 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 Sandpipers (Calidris pusilla) and Western Sandpipers (Calidris mauri) don’t really show that gape notch. 
Most field guides don't mention this feature.

Cape Cod, Massachusetts, USA, 9 August 2017