This paper introduces a non-invasive, quantitative technique to diagnose the progression of non-alcoholic fatty liver disease (NAFLD). The method is predicated on two fundamental principles: 1) the speed of sound in a fatty liver is lower than that in a healthy liver and 2) the quality of an ultrasound image is maximized when the beamformer's speed of sound matches the true speed of sound in the tissue being examined. The proposed method uses the echogenicity of an ultrasound image as a quantitative measure to estimate the true speed of sound within the liver parenchyma and capture its correlation with the underlying fat content. The proposed technique was evaluated in simulations and then tested ex vivo on sheep liver, mice liver (healthy and fatty) and tissue-mimicking phantoms. In the case of the phantom and sheep liver, the method was able to estimate the true speed of sound with errors of less than 0.5%; in the case of the mice livers, the method was able to accurately estimate the speed of sound within the livers (less than 1% error) and capture the correlation between fat content and speed of sound. Thereby, demonstrating the capability of ultrasound technology to non-invasively, quantitatively, and accurately diagnose NAFLD at point of care.