Good article from Birds.com
It will help you to identify the many Sandpipers.
Plus remember your field guide. I use Sibley.
Sandpipers are familiar to most birdwatchers. Yet their identification can be very frustrating. Most sandpipers are feathered in browns or soft grays, and gather in flocks that contain many species. Some are distinctively patterned, but others are so similar even experienced birders have trouble identifying them.
An important clue to identifying sandpipers is their bill shape. Most species have unique bill shapes, which help them specialize in capturing certain foods.
For instance, Sanderlings have very short bills compared to other sandpipers. It’s the perfect bill for grabbing tiny crustaceans that sit on the surface of the sand, when waves recede. Another sandpiper, the Dunlin, has a medium-length bill. This bill can probe a bit into the mud or sand, down to where little worms and insect larvae live. The Dowitcher’s bill is even longer- it can access worms that are out of the Dunlin’s reach. A Curlew, with its 9-inch-long, curving bill, gets the deepest prey of all- and the curve of the bill helps reach into crooked worm burrows.
Then there is the Avocet, whose unique, delicate bill turns up towards the tip. The Avocet uses it as a scythe. It swings it along the surface of the water, stirring up the mud and snapping up tiny crustaceans. The Turnstone’s bill is also upswept, but is much sturdier than the Avocet’s. The Turnstone uses it to flip over stones and beach debris, revealing the tiny prey hiding underneath.
All these different bill shapes can help the birdwatcher tell sandpipers apart. They also allow the birds to feed side by side- each species finding its own particular niche of food.