It remains debated whether galaxy mergers, gas flows, or internal processes shape the population of galaxies we see today. I will describe work to disentangle these issues using mock data derived from hydrodynamical simulations of galaxy formation. First, I use model IR and optical spectra to diagnose galaxy mergers, showing that the number of post-starburst galaxies is consistent with independent estimates of the galaxy merger rate. This owes to an underappreciated fact: not all major mergers are likely to produce a major starburst. Primarily, I will describe how we study galaxy morphology by building "mock observatories" around cosmological simulations. With these, we address how the common galaxy shapes emerged and how they relate to cessation of star formation in the universe. Early results suggest realistic populations of galaxy disks and bulges are a natural consequence of galaxy formation models with black hole and star formation feedback set to reproduce global quantities. Such simulations make readily falsifiable predictions that can be pressured by future surveys, and I will describe how we use these mock data to create diagnostics of galaxy dynamics.