A method and apparatus for identifying shim geometries. A first part and a second part are assembled with a couplant positioned between the first part and the second part at a shim location, at which a shim is to be installed, to form an assembly. A pulse of ultrasonic energy is sent into the assembly at each of a set of selected locations along the assembly corresponding to the shim location using an ultrasonic device. A plurality of echoes is detected for each of the set of selected locations using data generated by the ultrasonic device in response to the ultrasonic device receiving reflections of the ultrasonic energy. A geometry for the shim is identified based on the plurality of echoes detected at each selected location of the set of selected locations.