The histogram in Lightroom is very useful for watching for blown-out highlights or blocked shadows. It gives you a by-the-numbers feedback on the tones in your image. If you see parts of the histogram disappearing off the right or left, you know you’re clipping the highlights or shadows and losing tones.
But when using the histogram it’s not immediately apparent precisely which parts of the image are blowing out.
Lightroom has another very useful tool that will show you exactly where in an image the blowouts are happening. And whereas the histogram is separate from the image, this is an overlay over the actual image. So you can see not only that highlights are getting blown out of shadows blocked, but you can see precisely where on the image it’s happening.
Accessing it is simple. You simply hold down a key while you move the sliders in the Tone panel. On Mac, it’s the Option/ALT key. On Windows, it’s the ALT key.
The short version is that if you hold down the Option/ALT key while dragging the exposure slider, you’ll get indicators of where the blowouts are happening, with color-coded grading showing how much.
It works with all of the Tone sliders except for Contrast. The effect is much more subtle with the Highlights and Shadows sliders because those tools are specifically designed not to focus on the tonal extremes.
Here are some examples. This one is with the exposure slider:
This is with the whites slider:
And this with the blacks slider: