A good way to tell the difference between Mussids and Faviids is by running your finger over a closed deflated polyp. Mussids have irregular septa with large pointy teeth or lobes. Faviids have a smoother more uniform corallite with a more simple septal structure.
Usually larger corallites have larger teeth or lobes. This can be seen very easily in Scolymia, Cynarina, and Lobophyllia. But it is still evident in smaller polyps like Acanthastrea, Micromussa, and Blastomussa.