Spotted Redshanks are black in breeding plumage and very pale in winter.
In winter plumage they look quite similar to the Common 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.

1st & 2nd photo: In winter plumage. Texel, The Netherlands, 19 October 2018
3rd & 4th photo: In winter plumage. Zierikzee, The Netherlands, 17 January 2010
5th photo: in much darker summer plumage. Nieuwe Keverdijkse Polder, Weesp, The Netherlands, 24 April 2013