The little hash marks (like he uses in his signature) were the dead giveaway, but also the way some of the lines don't quite connect. Most people drawing that type of cartoon style, and especially for a coloring book, would make sure the contours all closed.
"The trouble with being a ghost writer or artist is that you must remain anonymous without credit.
If one wants the credit, one has to cease being a ghost and become a leader or innovator." — Bob Kane