To find the brightest highlights or the darkest shadows, using the Threshold command makes this a simple task. Here's how:
You can access Threshold mode while in the Levels palette, this way you can avoid going to the menu.
1. Choose Image>> Adjust>> Levels
2. The Levels dialog box will now appear.
3. Hold down the Alt/Opt key while dragging the Highlight and Shadow Input sliders and the image temporarily enters Threshold mode allowing you to see exactly where the darkest and lightest pixels reside.
4. Release the mouse button and the image reverts back to Levels mode.