In ACR (Adobe Camera Raw) if you hold the Alt key down as you select and move the Exposure slider, the screen will go black until you start to maximize the exposure value of the highlights. The maximized highlights will show up as colored dots to show which color channel is being blown first. When all the color channels are maxed out (blown) the dots will turn to white. If there are specular highlights or overexposed areas in the image as it was originally captured, they will be immediately noticable since the screen will not be completely black. Holding the ALT key down also works with the Recovery slider and the Blacks slider, thought when used with the Blacks slider the screen will turn white, not black.