VIEW YOUR LAYER MASK AS A RUBYLITH

If you want to view your Layer Mask by itself (rather than how the Layer Mask affects your overall image), hold the Option key (PC: Alt key) and click directly on the Layer Mask thumbnail in the Layers palette. This will display just the Mask itself. You can also view the Layer Mask like a Rubylith (a red overlay used in traditional masking) by pressing the Backslash key (\) on your keyboard.