The Least Sandpiper is the only of the "peeps" (the nickname for small sandpipers) with light-coloured (yellow/green) legs, the other ones have dark legs. You have to watch out for muddy legs looking light-coloured though. Also, they are the smallest, usually darker than Semipalmated Sandpipers (the other common peep) and have a longer, more curved bill than Semipalmated.
/Thomas