Thanks Chris.

Tl;dr: The current naive approach is by reading the rectangle size and position.

If you want to understand better:

• A footer is defined as a full-width rectangle at the bottom of the screen.
• A button is a rectangle within a specific range of width & height.
• A photo is a rectangle that's not anything else.
• A header is the same as a footer, but at the top of the screen.
• A card is the same as photo, but has no fill but has a simple border.