The following photos are of late season birds and I took them on South Cape May Beach in late October 2010; in brighter, August sun, both species will look much brighter and more spangled on the upperparts. If you intend going out to look for your own American Goldie, it is also as well to remember that this species is as likely to turn up inland on a sod farm or turned over farm field as on a coastal beach - perhaps more so.
A group of large plovers on the beach on a dull October day. From a distance, there is very little to choose from them? Which is the goldie? Well, of course you always go for the one in the middle of the picture because that is what the photographer was photographing! But it is clear to see here that there is very little to go on in overall color; goldies rarely look golden, except in full breeding plumage in spring while, conversely, black-bellies can actually look browner than might be expected.
Moving in with the scope, we can start to ignore the overall color similarities and home in on the subtle differences. Here a Black-bellied Plover is at left and an American Golden Plover right (with another black-belly in the top right corner) All these birds are juveniles and show pretty much the same color on the upperparts. To me the most obvious feature of the goldie is the dark cap, making the white supercilium over the eye look much more obvious. Next comes the noticeably smaller and finer bill, without the chunkiness of the black-belly bill. Other features may be more subtle, but still useful: the goldie has a more contrasting white throat due to the more heavily marked breast - which looks more checkered than streaked; the flanks look subtly barred not streaked and, of course the bird is smaller.
If the birds fly, you get a chance to home in on another useful feature - the famous black 'armpits' or axillaries of Black-bellied Plover (bottom bird), which contrast strongly with the white belly and remaining underwing. The top bird is our goldie, which shows a non-contrasting, dull grayish underwing. The third bird (on left) is a Dunlin, in case you were wondering!)
Another flight view, again showing that uniform gray underwing of an American goldie, but against a different colored background. Also interesting to note is the white underwing on the top left Black-belly, at an angle where the black axillaries cannot be seen - so it's not always straightforward!
Luckily the birds wheel round and land again - I didn't scare them off before anyone else saw them!! An upperside view in flight gives us another classic Black-belly feature - the large white patch on the tail and uppertail coverts. This feature can often be visible on settled birds too, particularly if preening. The American goldie (here on the right) is solidly dark in this area and it also shows a lot less white in the wing.