Put a leak light in the bell, then look down the inside of the body tube from the neck tenon receiver. The leak light will lightly illuminate the interior of the bore. Tilt the saxophone so you are looking straight down inside the, close one eye and sight down bore aligned exactly with one of the walls. Tilt it slightly. If it is straight, the whole wall should disappear from view at the same time. Repeat the procedure a few times until you have looked at all sides of the interior of the bore (N, S, E, W). If it is bent, wherever it is bent inwards from an external perspective will show as a high spot from an internal perspective- for instance, the lower half of the bore wall will disappear, but you can clearly see the G# tonehole means you've got a bend with its focal point around the G#. This example is for a simple, one-way bend. Obviously there can be multiple bends in different directions, bends on top of bends, and regular old dents.