Difficult from photos especially if the light is shining off it. But rounded corners where there should be sharp edges and reduced depth to the lettering are clues. Over polishing to a mirror finish before rebluing is another clue.
General rule of thumb if it looks too good to be true, then it probably is. Check out the sellers other guns - if they have identical finish then that's a good indicator too. I don't really know about Webley's, but I know their bluing was excellent.