Since the work of Lin and Shu in the 1960s the density wave theory, in several different forms, has been the leading candidate for a theory explaining the ubiquitous spiral arm patters seen in disk galaxies. Other theories exist, and rival versions of the density wave theory exist, but there has been little success in using observations to decide which theories are more successful. We present three such observational tests which we have been working on recently. Firstly we present a relationship between spiral arm pitch angle (which measures how tightly wound a spiral arm is), central bulge mass and gas density in the disk of a galaxy. Such a relationship is predicted by the original density wave theory of Lin and Shu. Secondly we show that spiral arm pitch angle varies between measurements made in optical versus infrared images in a way which is predicted by the density wave theory. Thirdly we examine the extent to which the number of arms in a spiral pattern varies with disk density, a prediction made by the swing amplification version of the density wave theory.