a) That's why they are a specialist. They are trained in the techniques to differentiate the fossil structure from the surrounding matrix (to the best of their ability). One of the biggest differences is texture; bone is very porous relative to most rock. Under a microscope that difference is much more apparent. You can also lick it because the porous texture of fossil sticks to your tongue more than the surrounding rock, but I doubt he did that.

b) Thats also why it took 7000 hours to do. It's really fucking hard, and he wanted to get it as right as possible. That means going very slow, taking a lot of care, and not taking shortcuts to put sculpting into play.