It takes a lot of experience and "reading" hundreds of hands. Basically you're checking how the thumb hinges in the lateral directions and trying to provide pitches that allow it to enter/exit the hole with little or no lateral tension. There are tests such as the Coke bottle test or picking up a pencil with your thumb and fingers that give some guidance, but I can tell more by manually manipulating the customer's thumb than by using any test. -- JohnP
