Choose Enhance > Adjust Lighting > Levels.
Choose Layers > New Adjustment Layer > Levels, or open an existing Levels adjustment layer.

Option-drag the Shadow slider to see
which areas will be clipped to black (level 0). Option-drag the
Highlight slider to see which areas will be clipped to white (level
255). Colored areas show clipping in individual channels.You can see the adjustment reflected in the Histogram palette.