Acoustic levitation devices use powerful ultrasonic standing waves to levitate objects in mid-air. We have created a system and method to measure the full harmonic content of the acoustic field accurately. Our study revealed that levitated particles alter the pressure field in a way that is not immediately apparent from the first harmonic alone, highlighting the importance of precise and non-invasive measurement techniques. Our method uses computational tomography and optical sensing with the acousto-optic effect and a 3 DOF mechanical scanner. Our experiment results in a detailed 3D pressure distribution map for each harmonic of the ultrasonic acoustic field that generates the levitation forces.