When identifying shorebirds, a very useful first step is usually determining what plumage it is in, something that is often relatively straightforward. All the small sandpipers are very plain overall in nonbreeding plumage, so this clearly isn't one of them. Juvenile plumage (seen on fall migration) is very bright and neatly marked above, never looking "messy", which this bird does. If you look at the back feathers on your bird, you'll notice a distinct contrast between some plain gray feathers and some bright rufous-coloured ones. This means our bird is in the process of molting. I'm not sure there's any easy way to tell if it's molting from juvenile or from adult breeding plumage without knowing what species it is.
The shape of this bird already narrows down the possiblities to the small
Calidris species - in Nova Scotia that means Sanderling, Dunlin, Purple, Pectoral, White-rumped, Baird's, Least, Semipalmated or maybe Western. See
http://ebird.org/ebird/GuideMe?cmd=deci ... &yr=all&m=As you noted this bird is smaller than a Semipalmated Plover, there are only 5 possibilities: White-rumped, Baird's, Least, Semipalmated or Western. Of these species, White-rumped and Baird's migrate to southern South America, and so have very long wings, extending well past the tip of the tail. You can see this very clearly in your photo. The other three have wings more or less reaching the tip of the tail.
At this point, separating White-rumped from Baird's is quite easy - the prominent supercilium, pink at the base of the bill, streaking on the flanks, rufous in the back, whitish appearance overall all point to White-rumped. Now you can look up the details and see that this is a adult molting to non-breeding plumage, based on timing (Sibley shows juveniles holding their plumage to November), flank streaking (absent in both juveniles and nonbreeding), and details of back pattern.
