Positioning the cursor over the triangles in the upper left and right of the Histogram panel will display areas of the photograph that are clipping to pure black or pure white. Areas that are clipping to black are shown with a blue overlay, areas clipping to white are shown with a red overlay.
Moving the cursor away from the shadow/highlight clipping icon hides the overlay. For a more constant display of the overlay, click the warning icon to show, click again to hide – or tap the “J” key to toggle on/off both of the shadow/highlight clipping previews at once.
An alternate way to preview clipping is to Opt (Mac) /Alt (Win) -drag the Exposure, Highlights, Shadows, Whites and/or Blacks sliders in the Basic panel. This differs from the clipping warnings above as you will see a per channel clipping preview displayed.
Note: In order to preview the clipping warning in ACR in Photoshop, tap the “O” / “U” key. Similar to Lightroom, clipped highlights will be displayed in red, shadows in blue. I remember these shortcuts by thinking of Over/Under exposure.