In winter plumage, the Common Redshanks look quite similar to the Spotted Redshank.
However, a Common Redshank's bill is approximately 1.5x the width of the bird's head whereas the Spotted Redshank's bill is approximately 2x the width of the head.
In addition, the Spotted Redshank's bill is more slender and has a very slight downward turn at the tip (like a hanging drop).
Also the feeding of the two species is different.
The Common Redshank feeds rather sedately, walking around picking at it's food whilst the Spotted Redshank move around much more quickly usually in deeper water than one would normally find Common Redshanks.

Brownsea Island, Dorset, England, 9 August 2009