The males tend to have a brow-band that goes all the way to the eyes and bolder and wider neck-bands (though the neck-band width changes with the stance of the bird and isn't a very good indicator of sex). The male’s bill tends to show more, and brighter orange and to have a sharp line between the orange and black.
Here are some examples of male Piping Plovers.
Females tend to have a brow-band that stops just short of the eyes and lighter and narrower neck-bands (the neck band is variable depending on the position of the bird so not a very definitive characteristic). Her bill-color tends to blend from orange to black.
Here are some examples of female Piping Plovers.
The coloration is a continuum and there are birds in the middle range that are difficult or impossible to sex by bill and plumage. Behavioral observations are needed to sex these individuals.
Atlantic coast Piping Plovers generally have much lighter markings and the breast band often isn't complete.
Atlantic Coast Piping Plover pair
Breeding behavior is another important clue. Though both sexes share equally in incubation and chick-watching duties, there are behavioral clues to sex. The males set up territories and attract females by doing flight displays, and they do most of the scraping. Both sexes are involved in territorial boundary walking and confrontations. If you observe tilt displays the male will usually be the one fanning out his wings, the male does the “goose-step” courtship display and, of course, if you observe copulation the male is on the female's back.
There are no sex differences in Piping Plovers' basic (non-breeding) plumage.