How do you evaluate user attention during UX testing?

I’m building UX HeatGrid, a Chrome extension that visualizes user attention and content density using a real time grid based heatmap.

It’s designed for quick UX checks while working on live pages, without setting up complex tests.

I’m curious
When you review a layout, how do you usually decide if users are actually focusing on the right areas?

Do you rely on clicks, scrolling behavior, visual balance, or something else?

Your answers help shape which signals and visuals matter most in UX HeatGrid.