Stand across Ocean Drive, in Lummus Park, and the contrast is the whole point.
Its neighbours are 1930s deco: horizontal, flat-roofed, smooth-rendered, with the ornament pressed into the surface rather than applied to it. Casa Casuarina is 1920s-idiom Mediterranean Revival: vertical emphasis, arched and colonnaded openings, deep window reveals, cast and carved ornament standing proud of the wall, a tiled roof, and a plan organised around an internal courtyard rather than a street frontage.
The specific things to look for:
The arcade and the arched entrance. The doorway is recessed and framed, which is a Mediterranean Revival habit and the opposite of the deco practice of flush, streamlined entrances.
The ornament density. Look at how much carved and moulded detail is carrying on above the ground floor. A deco building next door will have three stripes and a parapet. This has a full vocabulary.
The courtyard plan. You cannot see it from the street, but you can infer it from the way the building presents a wall to Ocean Drive rather than a bank of windows. The good rooms face inward.
The south wing. The 1990s extension, with the pool behind it, occupies the footprint of a hotel that was demolished to build it. Look for where the older and newer masonry meet.
The steps. Shallow, wide, and directly on the public pavement, which is unusual on this street and is why the entrance reads as a private house rather than a hotel door.