Actually, the most recent number that I heard was approx 85% of saddles don't fit. Fit depends on a lot of different things, including the horses level of training. The horses back changes when a rider is added to the equation. Does the back drop under the rider's weight? can the horse engage is core and lift the back under the rider? These will both change the pressure points in a saddle that is fitting when the horse is stationary.
As far as determining fit, how is the horses posture? Palpate the back, where are the tight muscles? Is it restricting movement in the shoulders? Is it extending past the last rib, interfering with the hip? How does the rider ride? Where is the rigging in relation to the heart girth? Even if a horse has a high pain tolerance, his back muscles and movement will not lie if saddle fit is an issue. What type of pad is being used? Has the pad been overused? Is it adequate? Is it too much padding if the tree is that close of a match to the back? Many things determine saddle fit.