Most Sandpiper species are pretty similar, specially as they come to Costa Rica in non-breeding plumage. Having a few different species mixed up in a single group can help for identification, as some of them will be showing the breeding plumage that distinguishes them, as well as other features like leg color. In this case, the Least Sandpiper has yellow legs, distinguishing it from the very similar Semipalmated Sandpiper. Also this bird is similar to the Pectoral Sandpiper, however the Pectoral is a lot bigger, has a yellowish bill, and the breast is streaked and delineated vs the white belly.