These two species are almost impossible to separate in non breeding plumage.
They used to be considered identical, Red-necked considered the Eastern version
Now that the birds are getting some color I tried to pick out a few.
Red-necked has slightly thickened tip of bill, medium sized primary projection and rather 'stocky' neck and shoulders and somewhat shorter legs.
In breeding plumage the throat is red/rufous while Little is white.
Little also is more complete red then Red-necked in breeding plumage.
Little has longer primary projection thus making them look more slender but taller as the tibia is longer. The bill is more fine and slightly drooping.
I hope I have identified these correctly.