This, is the type of "critical thinking" birding that requires patience. Focusing on behavior and structure - A couple of things to look for include the behavior, where WRSA, tend to wander out further into water than the smaller peeps. If you look at their feeding posture, White-rumped Sandpipers, tend to tilt forward as they feed and from a distance (easier seen) a careful observer will note the crossed wings a feature that the long winged peeps have and a distinctive field mark that could suggest either White-rumped Sandpiper or Baird's the other long winged peep. Review your field guides for other field marks to look for in identifying White-rumped Sandpiper.
Note: that a very chubby or fattened White-rumped Sandpiper, may not have that attenuated look, so work the bird carefully making sure it passes all the field tests before making the call. Here are a few photos of White-rumped Sandpiper in different plumage.
|White-rumped Sandpiper, Spring 2013 in Queens NY|
|White-rumped Sandpiper, Summer 2013 in Queens NY|