I got my info from the Benchmark maps book on Arizona. If you look really close you can see little "GWT" annotations in green in several places. Since these are pretty sparse, you then have to make an educated guess as to where the trail really is. I think the fact that the roads we took were GWT-marked for all of our trip that I can remember means were on one of the correct routes.
Here is a map of the GWT and one of our entire trip and a picture of what was probably the hardest spot. It was harder than it looks in the picture but not really all that hard and I would not hesitate to go up it the other direction.
Full trip Map
"Hard" spot coming down to Verde River