This duck was named for the chestnut ring between the black chest and purple-sheened head. This photo I took last May shows it quite well.
Late winter is a good time to see these birds in fresh plumage. Then you may look for the neck ring, especially on sunny days.
A few years ago the BirdFellow blog tackled this question.
We have also discussed this bird in more detail in At the pond... Ring-necked Duck.