Peeps are tricky even when in breeding plumage but throw in the complications of non-breeding plumage and you have some really difficult ID problems. Never the less, they can be told apart especially when offering crippling views. The below bird was photographed on the Fort Fisher Spit. Note the relatively white breast and face. Semis are dingier and the breast is usually unbroken or at least has less white. Bills can help if you have a definitive bird but there are a ton of birds with in between bills. Generally Semis have a more blunt tip to the bill and it is very thick at the base.