I cant show you a photo but it is normally a slight swelling just below the hock, not fluid but more bony. My girl has one but you wouldnt notice it unless you were a long sighted show judge!
Compare to the other hock - looking at the back edge of the hind leg, a normal hock drops from the point downwards in a pretty straight line. A horse with a curb will have a slight bulge just below the hock.
When they are new there is often heat and swelling. Once they have settled they are difficult to spot and rarely cause any problem.